Abstract: The energetic protons and electrons pervading the interplanetary space constitute the main source of radiation in the deep space missions, especially in the transfer orbits. Using the historical data of the IMP-8’s electron detection during 1973 through 2001, the statistical characteristics of the flux of the solar electron events are analyzed. It is shown that they follow a lognormal distribution. Assuming that the electron event occurs in a Poisson distribution, we establish an interplanetary electron flux model. By integrating the solar proton flux model, the orbit of the deep space mission, and the propagation law of the electron event in the heliosphere, the particle fluxes and the doses at different confidence levels are obtained. Then the failure probability of a certain device can be evaluated quantitatively based on its cumulative failure dose record. Taking the TL084 data acquisition module in the Mars mission with 8 month transfer orbit and 3 year Mars orbit as an example, the calculated failure probability is 1.01%.