According to the characteristics of type-B ultrasonic image with low contrast and SPECT image with low spatial resolution, an image fusion algorithm based on Nonsubsampled Shearlet Transform (NSST) and improved Pulse Coupled Neural Network (PCNN) is proposed. The NSST is used to decompose two registered source images, and low frequency sub-band coefficients and high frequency sub-band coefficients with different scales and directions are obtained. Low frequency coefficients are fused by the maximum of the regional energy. High frequency coefficients are fused by improved PCNN algorithm. The Sum Modified Laplacian is used for the input of PCNN, and the Energy of Gradient is used for the link intensity of PCNN, thus the high-frequency coefficients are selected by the sum of ignition output amplitude maximum. Finally, the fused image is reconstructed by inverse NSST. Experimental results demonstrate that the proposed algorithm achieves good results in the subjective perspective and objective criteria.