Using the method of density matrix, the rate equation of electron transport in mesoscopic systems are obtained from many-body Schrdinger equation. From those equations, we deduce expressions of spin current and charge current of single quantum well. It is found that there exists only spin current under certain conditions. Then we derive connection between the left current and the right one. The result shows that when the frequency of the rf field matches the Zeeman frequency of the electron in the quantum well, the spin current has a maximum value, which decreases with spin decoherence time.