The influence of the resonant helical field (RHF) with l=2 and n=1 on the m=2/n=1 natural tearing mode is studied numerically for large (A=20) and small (A=6) aspect ratio Tokamak. The behaviour of the nonlinear coupling among the tearing modes is different when the aspect ratio is different. However, it is found that in both cases RHF really suppresses the 2/1 mode when the widths of the islands generated by RHF and the 2/1 mode are the same order and the phase of RHF is opposite to the phase of the 2/1 mode. Primary mechanism of this suppression is the counter plasma flow induced by RHF. The growth rate of the 2/1 mode, however, is not remarkably affected by the existence of RHF, when the mode is not suppressed by RHF.