We establish a new quadrature detection system for self-mixing interferometry using two photodiodes and a 22.5-deg rotated beam splitter. The method is based on a rotating beam splitter placed between the laser diode and the measured object, and two quadrature self-mixing signals can be obtained. Then, an arctangent phase algorithm can be used to demodulate the quadrature signal to acquire the object vibration information. This method simplifies the self-mixing signal demodulation process and allows us to demodulate the vibrating displacement more easily. The experimental results demonstrate the feasibility of quadrature detection for self-mixing optical measurement. This Letter provides guidance for the design of self-mixing interferometers.