The influence of nonlinear absorption on cross interference correction in multi-gas analysis and modification method when gases are measured simultaneously are studied by non-dispersive infrared (NDIR) technique. The influence of interference coefficient variation caused by nonlinear absorption on system measurement accuracy has been deduced theoretically. The new interference equation modified from the conventional one uses the interference function to quantify the cross interference among the target gases. The interference functions vary proportionally with the nonlinear absorption. They can be obtained by least-squares fitting using the 3-order polynomials. Experiments show that the cross interference is corrected effectively by utilizing the fitted interference function even if the nonlinear absorption still exists in the system.