Abstract: A Bayesian reliability assessment method for spaceflight electronic device is put forward. The method combines the reliability information of the whole system and its subsystems. At first, the maximum entropy method is used to syncretize reliability information of subsystems; then the calculation formulas of moments of system reliability are deduced from reliability moments of subsystems and inheritance factor is determined and mixed Beta distribution is built; finally, the posterior distribution is used to assess system reliability. With the new method, the reliability of a spaceflight electronic device is evaluated to reach 0.9991 under the confidence level 0.70 with 240 whole system tests which is one-fifth of system tests needed by the classical method. It is feasible to use the new method to assess reliability of spaceflight electronic device.