In order to increase the security of the image encryption, the weighted fractional Fourier transform with dilation parameter (PWFRFT) is proposed. This PWFRFT is an extension of four-item weighted fractional Fourier transform (FRFT) defined by C.C.Shih. It owns four free parameters, which is called the vector parameter here, in the weight coefficients besides the order of the fractional Fourier transform. A novel optical image encryption algorithm is presented by the PWFRFT. The method owns ten secret keys when two-dimensional PWFRFT is operated one time: one kind is the order parameter and other kind is the vector parameter. Therefore, the new encryption method makes the security of image information better and the encryption process simpler. Digital simulations are presented to verify the validity and efficiency of the algorithm.