With the emergence of high speed optical communication system above 40 Gb/s, to ensure the quality of the transmitted optical signal, the optical signal in the transmission line should be monitored. The optical sampling technique is an important tool to perform time-resolved measurements of optical data signals at high bit rates with a bandwidth that cannot be reached by conventional photodetectors and oscilloscopes. The method with a fixed offset frequency can be used to sample the high speed optical signals with the sampling pulses of several hundred MHz. It relaxes the requirements of the bandwidth of electrical data processing system after the sampling. Based on the modeling and simulation of optical sampling process utilizing the sum-frequency generation in periodically poled lithium niobate (PPLN), the optical sampling of 10 Gb/s nonreture-to-zero (NRZ) and retrun-to-zero (RZ) signals in the transmission line obtained by Optsim is realized. The sampling results are processed by software-synchronized algorithm and the eye diagram of the signals is constructed. This method requires the least amount of hardware components. Compared with the sampling results of ideal AND gate, the sampling characteristics of PPLN waveguide and the quality of the sampling are analyzed by the obtained eye diagrams.