Laser heterodyne spectroscopy is a high-resolution remote sensing detection technology developed rapidly in recent years. Its device has the characteristics of small size, high spectral resolution, and is suitable for the detection of the concentration of atmospheric greenhouse gases. At present, it has been proved to be a practical and effective detection method in various observation experiments and has great application prospects and potentials in the field of earth atmosphere detection. Based on the existing laser heterodyne spectroscopy instruments, a new instrument structure is proposed in this paper. A fiber optic switch was used to modulate and split the direct sunlight, and realize the simultaneous detection of two-band laser heterodyne signals. It provides a new method for the next system integration of all-fiber laser heterodyne radiometer (LHR) and the construction of multi-band LHR. Based on the principle of laser heterodyne detection, this paper analyzes the advantages and key parameters of laser heterodyne spectrum detection technology. Combined with a self-developed high-precision solar tracker, a set of principle prototypes of a near-infrared dual-channel all-fiber LHR was built. The functions and parameters of the functional modules in LHR are elaborated in detail. The function principle and function of fiber optic switch are emphasized. The wavelength scanning mode and wavelength calibration method of the LHR are discussed. Based on this, the setting basis of related parameters and measurement method of the instrument function is discussed, and the instrument functions and the corresponding spectral resolution (0.004 4 cm^-1) of the LHR described in this paper are given. Using the built LHR to conduct actual atmospheric surveys in the Hefei area (31.9°N, 117.166°E), the laser heterodyne signals of CH₄ and CO₂ in the band of (6 056.2~6 058.1 cm^-1) and (6 035.6~6 036.5 cm^-1) were obtained simultaneously. The wavelength calibration and normalization of the absorption signals were performed to obtain the entire atmospheric transmittance spectrum of CH₄ and CO₂ molecules in the atmosphere. The signal-to-noise ratios of the measured spectral signals are 197 and 209, respectively and the spectral characteristics of molecular absorption signals are analyzed. The measurement practice in this paper shows that the fiber optical switch can be used to optimize the structure of laser heterodyne spectroscopy system, achieve the simultaneous measurement of multi-channel and multi-band LHR, and expand the application of LHR in the field of atmospheric detection.