This paper proposed a modulation recognition algorithm for M-QAM signals by the constellation diagram which does not require the prior information. Firstly, this scheme estimates the modulation parameters. Secondly, it reconstructs the received signals' constellation and uses kmeans cluster algorithm to compute the number of the signal constellation points which are as a recognition feature used for classification. The experimental simulation results proves that this method is effective for M-QAM signals, and the correct recognition ratio reaches 100% under the condition that the SNR is higher than 15dB. So it has relatively preferable practical value.