Using the quantitative error probability density method we studied the S/N ratio of alternately sampled signals digitized by a 4-channel A/D. A complete expression for the S/N ratio of a 4-channel A/D non-uniform sampling signal was deduced. First we obtained an expression for the S/N ratio of a 1-channel A/D uniform sampling signal when the sampling frequency was equal to or greater than 2 times the frequency of the sampled signal. Based on the S/N ratio of a 2-channel A/D,alternating,non-uniform sampling signal,we analyzed the distribution of quantitative error using the quantitative error probability density method and the distribution convolution formula. From this the S/N ratio expression of a 4-channel A/D sampling signal was deduced. The simulation result shows that the deduced expression is correct. Using the quantitative error probability density method we studied the S / N ratio of alternately sampled signals digitized by a 4-channel A / D. A complete expression for the S / N ratio of a 4-channel A / D non-uniform sampling signal was deduced. First we obtained an expression for the S / N ratio of a 1-channel A / D uniform sampling signal when the sampling frequency was equal to or greater than 2 times the frequency of the sampled signal. Based on the S / N ratio of a 2-channel A / D, alternating, non-uniform sampling signal, we analyzed the distribution of quantitative error using the quantitative error probability density method and the distribution convolution formula. From this the S / N ratio expression of a 4- channel A / D sampling signal was deduced. The simulation result shows that the deduced expression is correct.