3D pose estimation based on a monocular camera can be applied to various fields such as human-computer interaction and human action recognition. As a two-stage 3D pose estimator, VideoPose3D achieves state-of-the-art accuracy. However, because of the limitation of two-stage processing, image information is partially lost in the process of mapping 2D poses to 3D space, which results in limited final accuracy. This paper proposes an image-assisting pose estimation model and a back-projection based offset generating module. The image-assisting pose estimation model consists of a 2D pose processing branch and an image processing branch. Image information is processed to generate an offset to refine the intermediate 3D pose produced by the 2D pose processing network. The back-projection based offset generating module projects the intermediate 3D poses to 2D space and calculates the error between the projection and input 2D pose. With the error combining with extracted image feature, the neural network generates an offset to decrease the error. By evaluation, the accuracy on each action of Human3.6M dataset gets an average improvement of 0.9 mm over the VideoPose3D baseline.