This paper investigates the numerical computation of a Cauchy problem for L
aplace's equation which is a typical ill-posed problem. By using Green's fo
rmula, the problem is transformed to a moment problem. For numerical comput
ation of the moment problem, an error estimation and several numerical exam
ples for verification are presented. Necessary and sufficient conditions fo
r the existence of the solution of the Cauchy problems for Laplace's equati
on in two-dimension are also given.