In this paper, a fluorescent sensor for fluorescence immunoassay is presented. The assembled structure separates the path of emitted fluorescence from that of exciting light, hence the fluorescence is shined upon an N-Well/P-sub silicon photodiode and the exciting light is directed away from the photodiode. The N-Well/P-sub silicon photodiode has a metal mesh pattern which can suppress the incidence of long-wavelength light, such as infra-red light, into the photodiode and reduce dark current resulting from environmental long-wavelength light. The dark current is about 10pA. In this work, a read-out circuit of current-controlled oscillator using photo-current to control its oscillation frequency was designed to measure the fluorescent intensity. In the circuit, the output oscillation frequency is a linear function of fluorescent intensity. The experiment results show that the green fluorescent intensity can be measured with a good linearity of 99.2%.