The optical constants (refractive index and extinction coefficient) accuracy of thin films directly affects the properties of designed and fabricated optical devices. Most of the determination methods of optical constants are complex and cannot be applied during the film depositing process. In this paper, an optical constants determination method of thin films on-site is proposed. By monitoring the transmittance of depositing materials, this method can rapidly and accurately determine the optical constants on-site. For demonstration, the near-infrared optical constants of high-absorption material Si, low-absorption material Ta₂O₅ and ultra-low-absorption material SiO₂ are obtained as n=3.22, k=4.6×10_-3, n=2.06, k=1.3×10_-3 and n= 1.46, k=6.6×10_-5 respectively by this method. It reveals that this method is suitable for determining both strong and weak absorption materials" optical constants. It provides an effective way for precisely determining optical constants on-site, which is meaningful for the design and fabrication of high-quality optical devices.