An active laser triangulation method based on the telecentric lens is proposed to accurately measure the thickness of the object. The parallel projection method applies to the telecentric lens, and thus the relationship between the image point displacement and the object height change is linear. We establish a model of active laser triangulation based on the telecentric lens, and discusses the factors that influence the accuracy, including optical system error, machine error, and image processing error. In the experiment, we get the mean error less than 5 μm for thickness measurement when combing the active laser triangulation based on the telecentric lens and the compensation interpolation method.