In terms of the problem that polystyrene (PS) nanoparticles are hard to be aggregated and detected in water, this paper studies the mixed solution containing gold (Au) nanoparticles and PS nanoparticles, captures and detects the photothermal effect of PS nanoparticles by using a self-built optical manipulation-micro Raman system, and analyzes the photothermal effect of PS nanoparticles and the signal enhancement effect of the surface-enhanced Raman scattering (SERS) in the mixed solution. The results show that the movement speed of PS nanoparticles (80 nm) can be affected by the size and concentration of Au nanoparticles, and Au-PS aggregates with a diameter of 30 μm are formed in the photothermal trap. In addition, the SERS signal intensity of PS nanoparticles in aggregates is 7 times higher than that in the solution containing only Au nanoparticles, and it increases firstly before decreasing as the radius of Au-PS aggregates increases. The proposed method can capture the photothermal effect and detect the SERS signal of PS nanoparticles, with the signal intensity significantly improved and the detection limit reduced. Therefore, it is proven to be applicable in a variety of fields such as environmental pollution detection and nanodevice self-assembly.