We present a theoretical study of the in-plane electric filed induced exciton dissociation in two dimensional (2D) transition metal dichcogenides MX$_2$ (M=Mo, W; X=S, Se). The exciton resonance states are determined from continuum states by the complex coordinate rotation method with the Lagrange mesh method to solve the exciton Hamiltonian. Our results show that the exciton dissociation process can be effectively controlled by the electric field. The critical electric fields needed for ground state exciton to make the dissociation process dominating over combination processes is in the range of 73 - 91 V/$mu$m for monolayer MX$_2$. Compared with ground state exciton, the excited excitons are more easily to be dissociated due to their delocalization nature, e.g. the critical field for 2$s$ excited state is as low as 12 - 16 V/$mu$m . More importantly, we found that exciton become more susceptive to external electric field and a much smaller critical electric field is needed in the presence of a dielectric substrate and in finite-layer MX$_2$. This work may provide a promising way to enhance the exciton dissociation process and improve the performance of 2D materials based optoelectronic devices.
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