The invention relates to a multi-stage task spacecraft reliability improving method based on risks. The multi-stage task spacecraft reliability improving method based on the risks comprises the following steps of (1) determining the conversion relation of a key task stage and a task result state and between the task result states, (2) the key task stage risk characteristic is analyzed, a fault tree, a dynamic fault tree and a Bayes network method are used, a risk characteristic model is built, and relative data are collected to determine the key task failure occurring possibility and main risk factors, and (3) a spacecraft task process event tree model is built, the different task result state possibility is obtained by solving the model, the main risk factors are combined, and reliability improving measures are made. The multi-stage task spacecraft reliability improving method can realize the multi-stage task spacecraft conventional invalidity, redundancy backup invalidity and relative invalidity modeling analysis, the task result state possibility and the main risk factors are obtained, and the multi-stage task spacecraft reliability improving method based on the risks can provide technical support for the spacecraft reliability improving.