The experimental results of red-shift and blue-shift in absorption spectrum of Mo-doped ZnO are in mutual contradiction, and this phenomenon has not been explained rationally so far. For explaining this phenomenon, we analyze the energy band structure, state density, and absorption-spectrum distributions for each of Zn0.9583Mo0.0417O, Zn0.9375Mo0.0625O and Zn14Mo2O by first-principles calculation. The results show that within a limited doping amount range of 2.08 at%-3.13 at%, the higher Mo doping amount results in higher doping system volume, higher formation energy, lower system stability, and more difficult to dope. Meanwhile, all doping systems are converted into n-type degenerate semiconductors. Compared with the band gap of pure ZnO, the band gap of each doping system becomes narrow and the absorption spectrum shows red-shift. The higher the Mo doping amount, the weaker the narrowing of band gap becomes and the weaker the red-shift in absorption spectrum as well as the lower the electronic effective mass and the lower the electronic concentration; the lower the electronic mobility, the lower the electronic conductivity is; the lower the electronic magnetic moment is. The Curie temperature of doping system can reach a temperature higher than room temperature.