In this paper, an effective photoplethysmograph (PPG) acquiring and signal processing system based on square wave modulation is proposed. Through modulating the acquired signals on different frequency square waves, it is effective to get PPG signals with high SNR by using simple carrier wave generators and reduce the computation load and power. The proposed system includes an exquisite and wearable three way PPG front-end sensor for acquiring PPG signals, an FPGA-based DSP for signal modulation and an analog circuit with an efficient anti-aliasing filter for eliminating serious harmonic waves of the carriers. Finally, the proposed DSP is scheduled for chip fabrication using TSMC 0.18 um CMOS technology to accelerate the performance and reduce the cost. The computation load is reduced by 98.8% compared to conventional system without effective signal modulation and filtering. The proposed DSP operates at 6MHz and reduce 35.2% of power consumption with only 0.58 mW of power compared with the FPGA-based DSP. The proposed system is able to acquire stable PPG signals to accurately derive the physiological indexes of cardiovascular system such as heart rate, respiratory rate, oxygen concentration, etc..