A tractor electro-hydraulic control system was designed and studied based on the plowing depth system's principle of tractor hydraulic hitch mechanism. An electro-hydraulic proportional valve was used as the master valve in this system. The mathematic model of the tractor electro-hydraulic control system was set up and then the characteristic of load control and position control was analyzed. Finally experimental research was performed. The experimental results showed that transient time of position regulation is 0.65 s, and its static error is 1.5 cm; the adjusting time of draft regulation is 7.5 s; the fluctuating scope is ±1 cm, when the plowing depth is 20 cm in draft and position regulation. This system could meet the challenge of plowing depth control system in formidable natural conditions, and the object of uniform plowing depth and work stability were achieved.