Photodynamic therapy (PDT) is a minimally invasive method for treating oral leukoplakia. In this paper, we propose a portable PDT device consisting of a flexible circuit board with a liquid flow cooling module on the back. The light source size was 17mm×11mm×4mm, and the irradiation area of the light source was up to 100mm². The irradiance range of this device was from 10mW/cm² to 100mW/cm². Simulation and experimental results showed that the irradiance coefficient variation for a treatment area of 81mm² was less than 7%. At an irradiance of 100mW/cm², a device surface temperature of lower than 42^∘C can be achieved to satisfy the safety requirements under the conditions that the temperature of cooling liquid is 10^∘C and the liquid flow speed is above 12mL/min.Photodynamic therapy (PDT) is a minimally invasive method for treating oral leukoplakia. In this paper, we propose a portable PDT device consisting of a flexible circuit board with a liquid flow cooling module on the back. The light source size was 17mm×11mm×4mm, and the irradiation area of the light source was up to 100mm². The irradiance range of this device was from 10mW/cm² to 100mW/cm². Simulation and experimental results showed that the irradiance coefficient variation for a treatment area of 81mm² was less than 7%. At an irradiance of 100mW/cm², a device surface temperature of lower than 42^∘C can be achieved to satisfy the safety requirements under the conditions that the temperature of cooling liquid is 10^∘C and the liquid flow speed is above 12mL/min.