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In a time-varying fast-fading channel, due to the influence of amplitude fading and fast phase changing, the performance of a QAM system deteriorates seriously; there are also restraints on using high-dimension modulation in fast mobile communications. To solve this problem, many estimation methods have been presented. The process of channel estimation is practically that of compensating the channel characteristic. Thus, analyzing the performance of channel estimation methods also analyzes the equivalent channel after compensation. Characteristics of equivalent channel by compensation of some estimation is conducted in the course of equivalent channel estimation. Various channel estimations essentially estimate and compensate the channel with the channel correlation. The correlation of amplitude can be analyzed by amplitude ratio, while that of phase by phase difference. Hence, above all, we analyze the channel estimation performance by calculating the probability distribution of the amplitude ratio and phase difference at two random times. We name this method equivalent channel analysis method. By using this method, system performance is only the function of the Doppler shift and channel correlation coefficient that depend on the channel estimation method. We only need to calculate the correlation coefficient of two neighbouring symbols to obtain the relation between error probability and Doppler shift as used in channel estimation, thus simplifying the analysis of system performance.