The invention discloses a hydraulic transmission and automatic control stepless speed regulator, which comprises an enclosure, a speed regulator component, an overrunning clutch and a speed regulator component brake device, wherein the speed regulator component comprises an inner shell, a speed regulator input shaft and a speed regulator output shaft; a two-way hydraulic variable pump, a constant displacement hydraulic motor, a variable control driving device and a control system are arranged in the inner shell; the two-way hydraulic variable pump is connected in series with the constant displacement hydraulic motor; the two-way hydraulic variable pump, the constant displacement hydraulic motor and two oil passages communicated with the two-way hydraulic variable pump and the constant displacement hydraulic motor form a closed loop; a buffer cavity is connected to each of the two oil passages connected with the two-way hydraulic variable pump and the constant displacement hydraulic motor; in addition, a second hydraulic electromagnetic valve is arranged between an oil port of the two-way hydraulic variable pump and an oil port of the constant displacement hydraulic motor; a first hydraulic electromagnetic valve and the second hydraulic electromagnetic valve are electrically connected with the control system. The hydraulic transmission and automatic control stepless speed regulator is simple and compact in structure, low in manufacturing cost, continuous in transmission ratio, high in bearing capacity, high in transmission efficiency, safe, reliable and long in service life.