A wafer testing method adapted to test a thin wafer. The thin wafer is combined with a vacuum-release substrate to form a wafer-assembly, and the wafer-assembly is placed in a wafer cassette. The vacuum-release substrate is attached to a front surface of the wafer with an attaching force which is sensitive to air pressure. The method includes the following steps. First, taking out the wafer-assembly from the wafer cassette, then transferring the wafer-assembly to a warpage-detection-device and placing the wafer-assembly on a first stage of the warpage-detection-device. Then, detecting warpage of the wafer. If the warpage of the wafer is less than a warpage threshold, the wafer-assembly is taken out from the first stage, and the wafer-assembly is turned over to place the wafer-assembly on a second stage. Then, applying negative pressure to the vacuum-release substrate to eliminate the attaching force. Then, removing the vacuum-release substrate.