Stepless Capacity Control System (BHSCCS for short) is designed for reciprocating compressors and utilizes a partial-stroke, top-opening inlet valve. The system adopts a hydraulic system to control the mechanical unloader to press open the suction valve, control the compressor discharge volume by controlling the volume of air returning to the suction pipeline during the compression process, thus controlling the load of the unit to meet the demands of the production process on site, eliminating the return loss, effectively reducing the operating current of the unit under low load condition, and realizing the high efficiency and green operation of the unit.