Lubrication of high speed electric multiple unit(EMU) gear box is one of the key technologies of high speed train operation safety.Drain interval method of gear oils was proposed on the high speed EMU in order to study the decay characteristics for used gear oil and guide changing oil on site.In the test,6 Gearboxes of CRH6444 compartment on the Beijing-Shanghai line were monitored for 9 times，and the oil drain interval was estimated by four measures,physical and chemical properties evaluation method,contaminated properties evaluation method,additives consumption evaluation methods and wear characteristics evaluation method.The results show that it is appropriate for the used gear oil drain interval of high speed EMU gearbox with no more than 350 000 km on average.The research results provide technical reference for the high speed train-set gearbox maintenance and safe operation in China.