Photo-synaptic receptors emulating the natural biological photodetectors in human retina are required to build artificial visual perceptional systems as a new solution for augmented reality. Biological photoreceptors transform continuous light illumination into pulse-train like frequency-modulated signals. However, state-of-art photo-synaptic electronic devices are only capable of transforming light pulse signals into synaptic current signals without mimicking biological photodetectors. Here, a photo-synaptic pixel circuit (PSPC) for artificial retina with organic thin-film transistors (OTFTs) is reported to demonstrate a frequency-modulated spiking signals output responding to continuous light illumination. The reported PSPC is supposed to smooth the way for biomimetic photoreceptor arrays served as an artificial retina. Photosensing ability is achieved by building PSPC with phototransistors using IDTBT as the active layer. By applying continuous light illumination intensities ranging from 0 W/m2 to 2.7 W/m2 (to mimic the daily light intensity around 1 W/m2), the frequency of the photo-synaptic pixel circuit output is modulated from 416Hz to 990Hz, with a positive correlation between input light intensity and output signal frequency. This work provides a photodetector with continuous photo-sensing ability, biomimetically converting light intensity to frequency-modulated signals.