Based on a definitely integral formula, an expression of calculating atomic potential distribution function U(r) in a unit cell is derived as follows: Making use of this expression to calculate the phase grating in high resolution image simulation can greatly reduce.the calculating time. In this paper, the derivation of the expression is introduced, and then the computer routine is explained in details. Finally the potential projection map of Mg44Rh7 along [001] direction is shown as an illustration. All operations are carried out in real space, so we call the calculation method as the real space method. Based on a definite integral formula, an expression of calculated atomic potential distribution function U (r) in a unit cell is derived as follows: Making use of this expression to calculate the phase grating in high resolution image simulation can greatly reduce. The calculating time . In this paper, the derivation of the expression is introduced, and then the computer routine is explained in detail. Finally, the potential projection map of Mg44Rh7 along [001] direction is shown as an illustration. All operations are carried out in real space, so we call the calculation method as the real space method.