In order to obtain continuous spectral transmittance of total atmosphere with high accuracy, a sun-photometer is developed based on the combination of grating spectrometer and high precision two-dimensional rotating frame. We can use the wavelet transformation to achieve spectral denoising and baseline correction, and the design complexity of the instrument structure is reduced. A mixed calibration method which combines Langley calibration method and standard light source method is introduced in consideration of limitations of the two methods above mentioned. The base of calibration can be traced to the extra-atmospheric sun irradiance. A comparative experiment between the sun-photometer and the POM-2 photometer is carried out at the same place and time, and the subsequent comparative analysis is also made between data measured by the sun-photometer and data obtained by the MODTRAN software. The experiment indicates the reliability of instrument structure design and the high precision of the new calibration method.