At present, terahertz technology research mainly concentrates in its generation and detection mechanism. As the terahertz-wave being in the frequency range between microwave and visible light, whether the microwave and optical theory is applicable to terahertz-wave or has some common characteristic is to be studied. Analyzing and verifing the terahertz-wave field amplitude obeys the Gaussian distribution in the receiving plane perpendicular to the direction of propagation in the air medium by experiment; measurement brings forward energy distribution chart of terahertz wave in the air medium; the energy attenuation formulas of terahertz wave is deduced on the basis of the experimental data; the terahertz wavelength apparatus is designed using the Fabry-Perot (F-P) interferometer theory; the wavelength of emitted wave between 1~3 THz is measured from the SIFIR-50 THz laser of American Corehent Company. The related factors affecting the measurement precision including far-field divergence angle, the beam incident angle, mechanical vibration, temperature fluctuation and the refractive index fluctuation is analyzed at last.