The inverter-fed traction motor is one of the key components in the locomotive. However, the motor insulation failures have repeatedly occurred in premature. The failure mechanism for insulation under continuous impulses is different from that under sine waves. The single stress approach in design stage and also in ageing studies offers relative simplicity, but it is inapplicable to real life operating conditions which mostly are of multi-stress type. In this paper, a novel test apparatus of insulation failure time was designed for accelerating aging test. Meanwhile, electrical breakdown experiments on magnet wire insulating polyimide film (FCR 100) used in the inter-turn insulation of JD117 inverter-fed motor was conducted. The effects of multi-stress including impulse amplitude-temperature and impulse frequency-temperature were investigated respectively. The test results under different conditions were analyzed based on statistic analysis of Weibull distribution. Finally, insulation life models under combined thermal and electrical stresses were proposed using the multi-stress approach. A fitted formula for ageing life was presented which had two independent variables of voltage and temperature. The generality of the model lies in the fact that it can be used for designing of insulation system of all electrical components containing solid insulating materials surrounded by gas. Since the model is based on the probability of insulation deterioration under the action of the stresses applied, that gives us the opportunity to calculate the life time of insulation system for given system of stresses, the materials selected and the requested probability of failureless system operation.