This invention relates to a device and a method for monitoring and optimizing photothermal therapy, using a high-power continuous-wave laser beam and a pulsed laser beam, both transmitted through a single soft, multi-mode optical fiber with a diffuse active tip, to interstitially irradiate the target tissue at the same time. The continuous-wave laser light induces photothermal effect and increases tissue temperature and the pulsed laser light produces a photoacoustic signal. The photoacoustic signal intensity is used to monitor the temperature changes in the target tissue and to guide the irradiation of the high-power laser to optimize the photothermal effect by adjusting the light intensity and irradiation time.