Abstract: A certificateless signcryption scheme without bilinear pairing which proposed by Gao Jianxin et al. was analyzed. It was pointed that there was the error in the proof process of unforgeability, which leads to the insecure of the scheme. Therefore, an improved signcryption scheme was proposed. In terms of safety performance, the hash value of the partial public key generated by users was embedded into the index part of signcryption operations, as a result, the improved scheme can effectively resist public key replacement attacks. Moreover, in terms of computational cost, the improved scheme still has the high computing efficiency because of not using bilinear pairing.