In order to solve the problems such as bad versatility of image encryption and difficulty in directly encrypting non-square images which were caused by severe dependence on chaotic system, and also insufficient algorithm security because of the periodicity of the chaotic system, an improved encryption algorithm was put forward based on improved gravity model coupled with plain pixel correlation crossing mechanism. Firstly, this algorithm defined the combined chaotic transformation model by multiple 1D chaotic maps and output the key stream; and then, it designed pixel correlation crossing operator based on the location of plain pixels, and efficiently scrambled the plain image by combining the key stream; built dynamic variation function of mass to substitute the constant particle mass in the traditional gravity model and increase the sensitivity of the algorithm; and then diffused the permutation image with the improved gravity model to completely change the pixel value; finally it built an enhanced model to have secondary diffusion for the output cryptograph and increase its NPCR (number of pixels change rate) and UACI (unified averaged changed intensity). Compared with the current image encryption algorithm, the proposal could not only diffuse square images but also directly encrypt non-square objects, and it had higher versatility and security and stronger ability to resist differential attacks. The algorithm proposed can encrypt non-square images and have better versatility and security.