Polynomial-based calibration methods for a telecentric structured light system, composed of a camera with a telecentric lens and a pin-hole projector, can prevent measurement error introduced by projection calibration and thus have high measurement accuracy. However, using multiple calibration plate positions to cover the measuring volume increases the time to carry out the projection and capture the fringe images. To decrease the execution time and maintain high accuracy, we propose an improved calibration method for a telecentric structured light system based on the principle of projective geometry of a projector. Placing a calibration plate with ring markers in only three positions within the measuring volume, we can establish the conversion relationships between the phase and depth and between the pixel position and transverse coordinates. For validation, a telecentric structured light system was set up and calibrated with the proposed calibration method. The calibrated system was applied for three-dimensional shape measurements of a calibration plate and a cylinder surface. Experimental results demonstrate that the proposed telecentric structured light system calibration method has a measurement accuracy similar to the polynomial-based system calibration method.