A 3-D path detection system includes an image capture device, a radar device and a computing module. The image capture device is provided to produce a dynamic image for calculating the x- and y-direction (transverse) pixel-value displacements according to a captured moving object image. The radar device is configured to transmit an input wireless signal to a moving object and receive a reflection signal from the moving object, and is configured to calculate a z-direction (longitudinal) displacement of the moving object according to a Doppler shift in the reflection signal. The computing module is configured to construct a 3-D path of the moving object according to the x- and y-direction pixel-value displacements of the moving object image and the z-direction displacement of the moving object.