The quantum pure state and mixed state are the most important quantum states in quantum information. They are the foundation of studying density operator,quantum entanglement and quantum decoherence. The two states' definition and innate differences are given in quantum theory. The observable effect for quantum pure states' mutually phase is the most important characteristics in quantum interference. The transformation from quantum pure state to mixed state is analyzed through measurement of mechanic quantity ■z,■s and ■n,by quantum mechanics methods. The result shows that the measurement of mechanical quantity is anisotropic in the quantum pure state but for quantum mixed state is isotropic.