The surface of polymer materials is an important research area of surface and interfacial science. However, it is still challenging to detect the surface of polymer thin films in situ nondestructively and accurately at molecular level. In this study, a collinear phase-sensitive femtosecond sum frequency generation (SFG) spectroscopy, with the resolution of absolute molecular orientation, high phase stability, high signal-to-noise ratio, and the detectability of weak surface signal, was developed. A 35 fs broadband infrared laser was employed in this system, which has a wider spectral measurement range in one data acquisition. This system was applied to probe the precise structure (absolute orientation structure) of the surface of poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(hexamethylene adipamide) (nylon 6/6) polymer films. The results indicate that the ester methyl of PMMA, phenyl ring of PS, and N-H group of nylon 6/6 are orderly oriented on the surface and toward the air side, while other groups are relatively disordered or tend to lie down on the surface. The measurement results of different sample positions demonstrate the good experimental reproducibility and high phase stability of the system. Phase-sensitive femtosecond SFG is applied to the field of polymer film surface, which is of great significance for the understanding of the precise structure of polymer film surface and the optimization of polymer surface properties.