Abstract: RELAP5 is one of the most commonly used programs for reactor thermal hydraulic analysis. Due to the early developing time, RELAP5 does not have a friendly graphical user interface and interactive interface, resulting in the lack of human-computer interaction ability. In this work, the visual interactive simulation platform VITARS was developed by Java Script language based on Node.js framework. The origin RELAP5 was reformed then integrated into VITARS, and a bidirectional data communication pipe was established between RELAP5 process and VITARS process. In VITARS, a modeling module was developed, in which all the components in RELAP5 were abstracted into different pictograms and users could complete graphical modular modeling and parameter input for RELAP5 conveniently by stitching these pictograms, thus the use threshold of RELAP5 could be reduced. Due to the data communication pipe, VITARS could send control instructions to RELAP5 for pause, run, snapshot, terminate etc. The calculation results in RELAP5 could be displayed in VITARS as dynamic data meter or dynamic time-data curve in real time, and could be saved in VITARS for future viewing and extraction. In addition, a new signal logic computation system was developed in VITARS, in which more than forty kinds of signal logic computing components were built-in such as logical gate, proportional-integral, time delay integration, and flip-flop. As a data processing center for VITARS, the new signal logic computation system could read the thermal-hydraulic calculation results from RELAP5, user’s operation instruction etc., these data could be processed then distributed to RELAP5 to replace the thermal hydraulic boundary in time dependent component, adjustable equipment parameters or origin control data in the next time step, and could also be distributed to the data display system for real-time display. Therefore, with VITARS, complex proportion integration differentiation control system could be built for RELAP5, and user could adjust valve opening, pump speed and other equipment parameters in real time just like the reactor operators. Moreover, VITARS reserves the interfaces for coupling with other reactor analysis programs. VITARS also has the ability for users to customize the running interface. User could draw any reactor systems as the display background and kinds of graphical components such as dynamic data meter, warning light, reactor shut-down switch could be added. So that VITARS could also be used as a lightweight simulator of nuclear power plants. Model of Ling’ao Nuclear Power Plant was developed and a steady-state simulation was performed in VITARS. The simulation results were compared with that from origin RELAP5 program, which indicates the calculation accuracy of VITARS. VITARS could not only improves the efficiency of RELAP5, but also enhances human-computer interaction capabilities, which could be helpful for reactor safety analysis and operator training work.