In order to solve the problem of conveying logging tools from the surface into horizontal well,a stroke & retract-type tractor is designed.In virtue of overrunning clutch principle and three types of inchworm unit,continuous movement of the tractor is realized.By adopting hydraulic powered scheme,the tractor automatically unlock with the well wall in the event of power failure.The structural composition and working principle of the tractor are introduced,the walking mechanism is designed,and the schematic diagram of hydraulic system is presented.A virtual prototype of the tractor including hydraulic valve and pipeline is built in Pro/e and ADAMS software.The motion simulation on it is conducted,which proves the feasibility and rationality of the tractor.