A wafer carrier device capable of detecting positioning precision of a supporting plate thereof. In the present invention, a supporting plate connected to a mechanical arm has three reflectors and a positioning window. A detection device has a vertical signal generator to output a vertical alignment signal to a vertical signal receiver, and three level detectors to each output a second signal to the corresponding reflector respectively and receive a corresponding reflected level detection signal from the corresponding reflector. A determining unit determines whether the supporting plate is aligned with the vertical signal receiver according to the vertical alignment signal. The determining unit outputs a position rectification signal when the supporting plate is not aligned with the vertical signal receiver. A driver moves the supporting plate by mechanical arm to align the vertical signal receiver according to the position rectification signal.