The magnetic model of vertical cuboid was used to study the three dimensional analytic signal equation.The result shows that the three-dimensional analytic signal equation is an approximate relational expression,with the degree of approximation being in relation to the geometric parameters of the magnetic source.When the technique for enhancing the analytic signal is used to calculate the buried depth of the boundary of the magnetic source,the error depends on the radio between the horizontal scale of the magnetic source and the buried depth of its top surface.