Abstract: The accurate modelling of decay power release characteristics during pressurized water reactor (PWR) core operation represents a critical challenge for nuclear safety analysis and power regulation. Decay power constitutes a substantial element of core operating power, accounting for approximately 6% of the total power output and exhibiting an average half-life of approximately 10 minutes, which introduces significant time-dependent behavior during transient operations. Current reactor physics analysis methodologies predominantly neglect this temporal characteristic during operational periods, employing prompt release assumptions that merge decay energy release with instantaneous fission energy, while reserving delayed release models exclusively for post-shutdown scenarios. This limitation becomes particularly problematic during minute-level timescale power transients where decay power dynamics substantially influence core neutron flux distribution and power profiles. To address this technological gap, a comprehensive computational methodology for delayed release calculation of decay power during PWR core operation was developed in this paper. This methodology was integrated into NECP-Bamboo, a core physics analysis software independently developed by Xi’an Jiaotong University, primarily through modifications to the component calculation module Bamboo-Lattice and the core calculation module Bamboo-Core. The library of autonomous decay heat parameters was established through systematic processing of nuclear data, which results in the identification of eight key fission product nuclides (SFP group: ¹¹⁰Ag, ¹³⁴Cs, ¹³⁶Cs, ¹⁴⁷Pm, ¹⁴⁸Pm^m, ¹⁵⁴Eu, ¹⁵⁶Eu, ¹⁶⁰Tb) for individual treatment, while the remaining fission products were grouped into ten representative decay heat precursor categories. Within the Bamboo-Lattice module, the fission energy release cross-sections were reconstructed in such a manner as to exclude decay heat contributions, thereby generating modified few-group constants that serve to separate prompt fission energy from delayed decay components. The Bamboo-Core module subsequently implemented online calculation of decay heat precursor concentrations and their corresponding energy release through coupling with neutron diffusion equations and burnup calculations. Validation against the ANSI/ANS-5.1-2005 standard library demonstrates excellent agreement, with maximum relative deviations maintained below 0.7% across various burnup conditions. Two representative AP1000 power reduction cases reveal significant impacts of the delayed release model. During rapid 10 minute power adjustments, the conventional prompt release assumption introduces maximum relative deviations of −8.2% in core-average neutron flux and 10.15% in assembly-scale three-dimensional power distribution. The developed method demonstrates superior accuracy in capturing transient core behavior, particularly during localized intense power fluctuations where decay heat dynamics dominate short-term responses. This enhanced modeling capability provides critical support for minute-level reactor control strategies, load-following operations, and safety margin assessments. Future improvements will focus on expanding the precursor database for long-cycle accuracy and incorporating experimental validation data to quantify model uncertainties. The methodology establishes a foundation for next-generation reactor physics analysis with enhanced temporal resolution for dynamic operational scenarios.