The noise signals can be compressed and the optoelectronic signals can be amplified by the applied gate voltages in field-effect transistor(FET)-based photodetector. Organic materials have been widely applied in phototransistors, it is meaningful to fabricate all-organic photodetectors for large area, low cost and flexible device applications. However, multiple polymer layers are usually fabricated through solution processing and the 'solution corrosion' should be avoided during its fabrication process. Therefore, top-gate-bottom-contact(TGBC) FET configuration and orthogonal solvent were adopted in the experiments, and the bulty acetate was used as the solvent of poly(methyl methacrylate)(PMMA) to prevent destroying the poly(3-hexylthiophene)(P3HT) active layer. In this way, the FET-based all-organic photodetectors Au(Source, Drain)/P3HT(150 nm)/PMMA(800 nm)/Al(Gate) had been successfully prepared, showing an on-off current ratio of 103 and a maximum mobility of 8×10^-3 cm²·V^-1·s^-1. The photocurrent shown an obvious increment under illumination of a broad range of incident wavelengths from 350 nm to 650 nm, giving a maximum photo-to-dark current ratio of 75 under 0.1 mW/cm2 illumination. The largest photoresponsivity of ~0.28 A/W was obtained under the illumination of 600 nm, and the trend of photoresponsivity corresponds to that of the absorption of P3HT film.