A continuous non-stationary load spectrum of wheel loader transmission was obtained from field testing by a special test system, including time-history of torque, speed, oil pressure and gears. During the data processing, each work cycle was divided into six steps: no-load forward, shoveling, heavy-load back, heavy-load forward, unloading and no-load back with considering the characteristic of gears, torque and oil pressure signals. Discretization, Butterworth filter and the singularity rejecting were adopted to analyze the load data of each step, from which the non-stationary random signals were translated into the pure stationary load signals. The time-history load testing method and data processing of random load spectrum of transmission was presented.