In general, two dimensional dynamic NMR spectrum can be obtained by means of three-pulses experiment. But the informations from this experiment may sometimes be obscured if the scalar coupling in the nuclear system appears. In this work, a theoretical 2D dynamic NMR spectrum of weak coupled two spin system is calculated in Liouvelle space. The analytic solutions of density matrix in AX(I=1/2) system is given. It is shown that (1) after applying of gradient pulse, a z-component, a zero-quantum coherence and a double-quantum coherence will be left in the density matrix; (2) the effects of scalar coupling is different from those of exchange and relaxation and the former only contributes to the zero-quantum coherence and double-quantum coherence. Therefore, one can expect that the interference from the scalar coupling would be eliminated in some appropriate experiment.