To evaluate human workload during manual operation, 9 subjects participated in the attitude maintaining experiment with simulated earth scenery as the display and a stick as the controller. Mean-rectified EMG was chosen as the index. Surface EMG was recorded in a couple of agonist and antagonist. Kinematics equation was simulated on a analog computer, and three difficulty of work were controlled by changing the amplifier's coefficients. 486 Computer was used to record the quantity of subject's operation and the data of Mean EMG from amplifier. Relevant software was compiled. The results showed that there was a linear relationship between mean-rectified EMG and difficulty of work. The effects of training on workload were also discussed. Mean EMG can be used as an index for evaluating human workload.