In order to improve the cooperative communication quality, this paper proposes an opportunistic cooperation scheme with adaptive Decoding-forward (DF) relaying, and analyzed the system performance of this scheme by establishing a general mathematic probability model. Based on the derivation of the system probability density function in Rayleigh fading channels, the closed-form expressions of the average spectral efficiency, outage probability and average bit error rate were obtained with the system in link adaptation. Monte Carlo simulations have shown that this adaptive decode-and-forward opportunistic cooperation outperforms previous systems and has about 30% higher average spectral efficiency than the Amply-and-forward (AF) cooperative system.