The nanofabrication platform was carried out using an atomic force microscope (AFM) system and a continuous wave (cw) laser to investigate the influence of laser power on the underlying mechanism of nanostructures fabricated by multiphysics fields tip enhancement (MFTE) induced by a cw laser irradiating the AFM probe tip. The nanostructure fabrication of nanopits and grooves and nanodots and lines on a polymethyl methacrylate thin film was conducted in an ambient environment by changing the incident laser power. The dependence of the MFTE on laser power was numerically analyzed, too. The lateral dimensions of nanopits and grooves and nanodots and lines characterized in situ were 154 nm, 96 nm, 188 nm, and 25 nm, respectively, breaking the optical diffraction limit. It turned out that the nanostructures converted from craters (pits and grooves) to protrusions (dots and lines) when altered with the laser power. Different laser powers can trigger the MFTE to change, thus, inducing varied coupling energy, which is the essential mechanism for nanostructure conversion. We also established a model to analyze the nanostructures transition and to predict the dimensions of nanostructures. The simulation results demonstrate that the MFTE has an essential effect on the formation of nanostructures, which are in good agreement with the experimental results.