The paper presents a method to use the photothermal phase difference to map metal nanoparticle (NP) reagent with optical coherence tomography (OCT). The depth-resolved, real-time, and highly localized light-to-heat conversion can be traced by recording the photothermal phase. For adiabatic absorption, the photothermal phase difference represents the localized light absorption characteristics of the NPs. In order to prove the effectiveness of our method, we firstly used FD-OCT imaging system to do agar phantom experiments with different concentrations of NP reagent. The results show that the photothermal phase difference can be used to distinguish different concentrations of NP reagent in agar model. Subsequently, a 3-month-old male C57BL / 6J mouse weighing about 25 g was used to verify the feasibility of our method for reconstructing the distribution of NP reagent in tissues. Before the experiment, 0.12 ml chloral hydrate with a concentration of 0.15 g/ml was used for anesthesia, and then 100 ml NP solution with a concentration of 60 μg/L was injected into the bladder of mice through a trocar inserted into the urethra. After about 4 hours, the mouse bladder was used to reconstruct the photothermal image. By analyzing the OCT structural image of mouse bladder, we can clearly observe two distinguishable layers. The first layer is detrusor and the second layer is muscularis mucosa. The results are consistent with those reported. In conclusion, we have proved theoretically and experimentally that the distribution of NP reagent in tissues can be reconstructed by photothermal phase difference. Owing to thermal expansion and thermal refractive index effect, the photothermal phase difference can produce local temperature changes in the tissue. For adiabatic absorption, the photothermal phase difference is related to the concentration of NPs. Therefore, it has been used to distinguish different concentrations of NP in the model and reconstruct the distribution of NP in the bladder of mice.