A tunable optical rail is embedded into the cavity of a nonlinear-polarization-rotation (NPR) mode-locked fiber laser to generate a sampling pulse with different repetition frequencies and realize bit-rate-adaptive software synchronous optical sampling. Two ultrashort pulses (20.26677 and 20.22900 MHz) are derived, and a 100-MHz data signal is sampled twice with these pulses based on sum-frequency generation (SFG) in periodically poled lithium niobate (PPLN). The eye diagram is successfully recovered, and an estimated bit rate of 102.22 MHz is derived. This method is feasible for bit rates ranging from 200 MHz to 1 GHz, with <3% relative error.