The widespread use of the underwater camera provides an effective nondestructive means for underwater measurement in various scenarios. The underwater camera captures objects through at least one refraction by the interface between the water and the protecting house. It is a non-single viewpoint (non-SVP) imaging system and the assumption of single viewpoint (SVP) camera model is invalid. Always, we must calibrate the camera system to quantify the image deformation caused by refraction and to measure the object accuracy. However, it is sometimes difficult to be done. In this paper, we propose a novel flexible underwater camera model. Then, we present an underwater camera calibration method based on the proposed camera model to calibrate the underwater camera. Both synthetic and real experiments validate the proposed method.