Interstitial laser immunotherapy (ILIT) is designed to use photothermal and immunological interactions for treatment of metastatic cancers. The photothermal effect is crucial in inducing anti-tumor immune responses in the host. Tissue temperature and tissue optical properties are important factors in this process. In this study, a device combining interstitial photoacoustic (PA) technique and interstitial laser photothermal interaction is proposed. Together with computational simulation, this device was designed to determine temperature distributions and tissue optical properties during laser treatment. Experiments were performed using ex-vivo porcine liver tissue. Our results demonstrated that interstitial PA signal amplitude was linearly dependent on tissue temperature in the temperature ranges of 20 600C, as well as 65 800C, with a different slope, due to the change of tissue optical properties. Using the directly measured temperature in the tissue around the interstitial optical fiber diffusion tip for calibration, the theoretical temperature distribution predicted by the bioheat equation was used to extract optical properties of tissue. Finally, the three-dimensional temperature distribution was simulated to guide tumor destruction and immunological stimulation. Thus, this novel device and method could be used for monitoring and controlling ILIT for cancer treatment.