Abstract: Nuclear data are the important source of uncertainties that affect nuclide inventory analysis. The new domestic code GNET (general nuclear engineering tool) has been developed to study the uncertainties of nuclide inventory and relevant quantities due to nuclear data. GNET is a pure Python3 package devoted to promote and accelerate the application in nuclear data, nuclide inventory, reactor physics data analysis, and so on. The code makes use of HDF5 format file and provides several auxiliary library files. The uncertainty quantification method in GNET is based on stochastic sampling. Nuclear data and their uncertainties such as decay data, fission yield and cross sections have been sampled to generate random library files. Then the application calculation with these files was performed. Finally, a statistical analysis of the response variables can be carried out to obtain mean values and their standard deviation. Uncertainties and covariance data in ENDF/B-Ⅶ.1 were analyzed. Nuclear data libraries lack fission product yield covariance data. To solve the problem, the Bayesian updating method was applied to obtain fission yield covariance. As one of state-of-art fission yield covariance generation methodologies, the Bayesian updating method is an adjustment technique which updates the prior data with new knowledge. The covariance matrix of independent fission yields has been generated using consistency with cumulative fission yields, binary fission, mass and charge conservations. In this paper, the decay heat uncertainty quantification for a burst thermal fission of ²³⁵U was investigated with GNET. Decay heat was calculated by summation method. The result of decay heat for a burst thermal fission of ²³⁵U was compared with FISPACT-Ⅱ results, ORNL and Tobias’ compiled data. It is shown that GNET result is consistent with FISPACT-Ⅱ and measurement results. A large number of calculations were used to calculate decay heat uncertainty due to uncertainties of decay constant, branching ratio, decay energy and IFYs in ENDF/B-Ⅶ.1. The convergence of uncertainty calculations was tested, which shows that it is sufficient to achieve an acceptable statistical stability using about 200 random files in this problem. The decay heat uncertainty as a function of the cooling time for a burst thermal fission of ²³⁵U was given. The total relative decay heat uncertainty is less than 6.5%. The uncertainty of fission yields is the major contributors to total decay heat uncertainty, followed by uncertainties of decay energy and decay constant. Uncertainty of branching ratio has very weak effect on total decay heat uncertainties and can be ignored. The preliminary application shows that the flexible code GNET has implemented uncertainty quantification capability. It is planned to extend the code to uncertainty calculations of more physical quantities in nuclear engineering simulations.