The unconventional photon blockade(UPB)for low-frequency mode is investigated in a three-mode system with double second-order nonlinearity.By analyzing the Hamiltonian of the system,the optimal analytic condition of UPB in low-frequency mode is obtained.The numerical results are calculated by solving the master equation in a truncated Fock space,which agrees well with the analytic conditions.Through the numerical analysis of the system,it is found that the weak driving strength is favorable for the system to realize the UPB effect,and the system is insensitive to the changes of attenuation rate and environmental temperature.The comparison with the two-mode system and another similar three-mode system shows that,under similar system parameters,the UPB effect of this double two-order nonlinear system is more obvious.