The mirror nucleui emissivity in medium energy heavy ion collisions was studied by using a quantum molecular dynamics model. Four stable light nuclide reaction systems (⁶⁴Ge+⁶⁴Ge，⁶⁴Zn+⁶⁴Zn，⁶⁴Ni+⁶⁴Ni，⁶⁴Fe+⁶⁴Fe) and five stable heavy nuclide reaction systems (¹²⁴Ba+¹²⁴Ba，¹²⁴Xe+¹²⁴Xe，¹²⁴Te+¹²⁴Te，¹²⁴Sn+¹²⁴Sn，¹²⁴Cd+¹²⁴Cd) were selected to study the change of the characteristics of the mirror nucleui emissivity at different time, under different incident energy and with different system neutron-proton ratio. The results show that both the Y(n)/Y(p) ratio and the Y(³H)/Y(³He) ratio increase with the increase of the system neutron-proton ratio in both light and heavy reaction systems. Y(n)/Y(p) is the ratio of pre-equilibrium neutron yield to proton yield, Y(³H)/Y(³He) is the ratio of pre-equilibrium ³H yield to ³He yield. Compared with the Y(n)/Y(p) ratio, the isospin-dependent mean field is more sensitive to the Y(³H)/Y(³He) ratio. In the heavy reaction system and neutron deficient reaction system, the mirror nucleui emissivity Y(³H)/Y(³He) is more suitable as a sensitive probe for isospin-dependent mean field.