Regarding the board in spacecraft with instrument and instrument-box installed, the mathematical model for heat analysis of the board is built and numerical calculation is conducted. The calculation results show the temperature distribution of the board when one or two instruments are installed at various places or generate various amount of heat et al. The resources which aggravate uneven temperature distribution can be found out by the calculation, and the temperature conditions that the instruments work in are also obtained. At the same time, the results can be regarded as an important reference for heat-controlling design of spacecraft, and it offers a guide for the nodes locations in the further heat-analysis.