Micro-nano particle sorting is significant in biomedical engineering, environmental engineering, and chemical molecular screening. The sorting of micro-nano particles needs to control different kinds of particles from disordered states to their respective positions. This process requires the force acting on the particles to control their movement and trajectory. With the vigorous development of microfluidic technology, various sorting technologies have emerged. The thermal field manipulation method is one of the most promising approaches because of its optic-free mechanism, ease of integration with other functional units, no vibration, and no limitation for the medium conductivity. We proposed a new approach to realize micro-nano particle sorting using thermophoresis induced by the photothermal effect. And separation characteristics were studied using numerical simulation by considering the fluid flow heat transfer and particle migration behavior. Firstly, the force, including buoyancy, viscous resistance, thermophoresis, lift, pressure gradient force, and virtual mass force, was analyzed on particles of 0.5 and 5.0 µm. And it was found that the resultant force of thermophoretic force and viscous resistance was dominant in the separation process. Secondly, a new sorting microchannel structure based on the photothermal effect was designed according to the forces of the particles. And this proposed structure can realize effective separation between 0.5 and 5.0 µm particles. At last, the impact of laser power, laser beam radius, laser-focused position, inlet velocity, and outlet width on the sorting performance were studied. The separation distance between 0.5 and 5.0 µm particles was wider for higher laser power, smaller laser beam, fronter laser position, lower inlet velocity, and wider outlet width are conducive to particle separation. This sorting method proposed in this paper has the advantage of being low-power, non-invasive, and dynamically controlling.