The formaldehyde(HCHO) detecting at room temperature is of great significance.Different ratios of P3HT/ZnO composite films(3:1,1:1,and 1:3) were deposited on the organic thin film transistor(OTFT) by spray-deposition technology,and the electrical properties and HCHO-sensing properties of all the prepared OTFT devices were measured by Keithley 4200-SCS source measurement unit.The results show that the OTFT sensor based on the P3HT/ZnO films with the ratio of 1:1 exhibited the best output and transfer curves.Different changing tendency were observed with the increase of ZnO proportion when exposed to HCHO at room temperature,and the device with the ratio of 1:1 behaved a good response and recovery characteristics. The formaldehyde (HCHO) detecting at room temperature is of great significance. Different ratios of P3HT / ZnO composite films (3: 1, 1, and 1: 3) were deposited on the organic thin film transistor (OTFT) deposition technology, and the electrical properties and HCHO-sensing properties of all the prepared OTFT devices were measured by Keithley 4200-SCS source measurement unit. The results show that the OTFT sensor based on the P3HT / ZnO films with the ratio of 1: 1 exhibit the best output and transfer curves. Different changing tendency were observed with the increase of ZnO proportion when exposed to HCHO at room temperature, and the device with the ratio of 1: 1 behaved a good response and recovery characteristics.