We investigate the ultradeep subwavelength imaging of a superlens with a surface plasmon polariton (SPP) cavity. A silver layer is added in the imaging region of the superlens to form an Ag film lens/photoresist/Ag layer cavity, in which the long-range plasmon mode is drastically suppressed and the field of the imaging is significantly amplified and extended over to the entire imaging region due to the SPP resonance inside the cavity. Results show that much improved quality of image with much suppressed sidelobes and much extended depth of focus can be obtained with the cavity structure when compared with the conventional open structure. This is confirmed by the transfer function of the system, which becomes flatter with the cavity structure. The proposed method provides a novel and practically feasible way to achieve images with both high resolution and large depth of field.