Based on the infinite homography relation between the vanish points on images captured in different camera positions and the relative position information, a self-calibration method is proposed to calibrate the focal-length of camera accurately. By projecting parallel lines in two positions, this method achieves a pair of vanish points in image plane, an infinite homography constraint is established and the focal-length is solved. Considering the parallel extent of lines that connecting corresponding optical centers and vanish points as the optimization index of Nelder-Mead nonlinear simplex method, focal-length optimization is achieved, the effect of image noises and rotation uncertainties are restrained effectively, and the accuracy of calibration result and algorithm robustness are improved greatly. Plenty of simulations show that this method proposed is both accurate, robust and real-timed.