The all-optical sampling based on the nonlinear polarization rotation in semiconductor optical amplifiers has the advantages of high-speed operation and small pump power, and it shows a possible solution for high-speed all-optical analog-to-digital (A/D) conversion. A novel method to characterize the linearity and efficiency of the all-optical sampling is proposed. The all-optical sampling transfer curve is fitted by a polynomial function, and the sampling system is described by the polynomial function. The efficiency and the harmonic distortion of the sampling process can be analytically expressed by the polynomial coefficients after substitution and expansion of the cosine function. It is not necessary to execute a complicated Fourier transform, and the results from our method are agreeable with those obtained by the Fourier analysis method. This method is simple but accurate and is useful for the full characterization on the linearity and efficiency of all-optical sampling based on semiconductor optical amplifier and can be extended to the nonlinear distortion of the analogue signal modulation.