In order to study the influence of underwater thermal disturbance environment on imaging distortion, such as optical imaging distortion or imaging blur, the level of distortion of target image in radial and axial directions was evaluated by using the gray scale distribution, structural similarity image measurement (SSIM), and normalized maximum gray-scale gradient definition evaluation function of underwater images. Furthermore, the laws of underwater thermal disturbance on optical imaging changes were obtained. Experimental results show that with the increase of the axial distance between the imaging system and the target, the level of image distortion and blurring becomes larger and larger. When the axial distance is equal to 500 mm, the SSIM is better than 0.7 and the normalized definition is better than 0.8. When the axial distance reaches 1500 mm, the SSIM is lower than 0.2 and the normalized definition is less than 0.6. In addition, when the axial distance equals 500 mm, the drift of the edges will be greater as the imaging area comes closer the heating source in the radial direction, that is, the imaging distortion is more serious. Finally, under the same axial and radial conditions, the conclusion that the SSIM and normalized definition values of the target images are different at different times can provide a reference for further underwater image restoration.