The invention relates to the technical field of PCBs and especially relates to a PCB online semiconductor device testing system. The system comprises a placement table, a sliding rod, sliding sleeves,a telescopic spring, a movable rod, a limiting plate, an insertion column, a sliding rail, a sliding block, an extrusion spring, a limiting column, a rope, an adjusting screw rod, a locking nut, a guide rail, a movable block, a needle bed and a testing system. A sliding groove is arranged in a bottom of an inner wall of the placement table, middle positions of the two sides of the inner wall of the sliding groove are fixedly connected with two ends of the sliding rod correspondingly, the two sliding sleeves are connected with an outer surface of the sliding rod in a sleeving mode correspondingly, and the sliding sleeves are slidably connected with the sliding rod. In the invention, a purpose that the system is suitable for clamping and positioning PCBs of different models and sizes is achieved; and meanwhile, an overall structure is simple and convenient, automatic change can be performed according to the size, and a mode that clamps need to be replaced according to models in the pastwhen the PCB is detected is replaced so that detection efficiency is improved, a detection procedure is simplified, and a clamping requirement in a PCB detection process is met.