The invention relates to a heavy double-column vertical lathe cross beam gravity deformation prediction method based on a finite difference method. Due to the fact that an existing finite element analysis computing method cannot accurately calculate a cross beam gravity deformation curve on the condition that actual materials have not uniform attributes, the calculation result much differs from an actual deformation value. The heavy double-column vertical lathe cross beam gravity deformation prediction method based on the finite difference method comprises the steps that actual assembling conditions are simulated to design a heavy machine tool cross beam self-weight deformation experiment to obtain a self-weight deformation curve. By means of the material mechanical theory, a cross beam is simplified into a simply supported beam mechanical model and then made into micro segments through discretization, and a cross beam gravity deformation discretization model is built on the basis of the finite difference method; the equivalent weight bending rigidity of each discrete micro segment is calculated; the cross beam finite element gravity deformation curve is calculated; the equivalent weight bending rigidity is utilized for correcting the cross beam finite element gravity deformation curve on the basis of the finite difference method to obtain a final cross beam gravity deformation curve. The heavy double-column vertical lathe cross beam gravity deformation prediction method is applied to heavy double-column vertical lath cross beam gravity deformation curve calculation.