It is beyond common understanding that the photoluminescence lifetime of the transition T-4(1)-->(6)A(1) Of Mn2+ doped in ZnS decreases dramatically by five orders of magnitude from bulk to nanocrystal since the magnetic interactions which usually make the spin-forbidden transition allowed cannot be so strong. In this paper, we present a possible mechanism involving the exchange Coulomb interaction between the d electrons of Mn2+ and the electrons of the host. Assuming that the spin of the ground state of the host is not zero, and that the exchange Coulomb interaction causes mixing between the excited T-4(1) State of Mn2+ and a certain excited state of the host whose energy is slightly higher than that of the T-4(1) state of Mn2+, we demonstrate that the spin-forbidden transition could be almost allowed. Moreover, the mixing degree increases as the particle size of the nanocrystal decreases. A numerical estimation about this mechanism agrees with the experimental results of optical and magnetical properties