A testing system for testing semiconductor package stacking chips is disclosed. The system includes a testing socket, a testing arm, and a testing mechanism. The testing mechanism includes a probe testing device. The probe testing device has a testing chip inside and a plurality of testing probes electrically connected to the testing chip. The plurality of testing probes extends toward the testing socket for contacting a chip-under-test loaded on the testing socket. When the testing mechanism moves to an upper position between the testing socket and the testing arm, the testing arm moves downward in the vertical direction and presses down the testing mechanism thereby coercing the plurality of testing probes in the testing mechanism to closely abut against the chip-under-test, so that the testing chip inside the testing mechanism can electrically connect to the chip-under-test for forming a test loop.