An optical signal detection apparatus and method for a computer mouse compares two output signals of a first photo-transistor and a second photo-transistor to obtain a reference signal. Then a pulse signal generating unit utilizes the reference signal to generate a series of pulse signals each occurring when the voltage level of the reference signal is changed. Thereafter a sample and hold circuit samples and holds the voltage value of the output signals of the photo-transistors at the moments that pulse signals are generated. A dynamic reference voltage generating unit uses the sampled voltage values to calculate a dynamic reference voltage. The voltage level of the dynamic reference voltage is varied with the output signals of the photo-transistors. By using the dynamic reference voltage to compare with the output signals of the photo-transistors, the on/off statuses of the photo-transistors can be precisely detected.