Fines migration has posed a great challenge to gas and water production in CBM reservoirs, resulting not only in dramatic permeability reduction but also in excessive wear on equipment. The objective of this study was to investigate critical flow conditions for massive fines detachment in the dewatering phase, for the purpose of yielding an improved understanding of fines detachment mechanisms and their effective control in the field. First, fines migration experiments under saturated conditions, including effluent concentration and permeability measurements, were conducted at elevated pressure gradients on fractured coal samples with various apertures. Experimental results indicate the existence of a critical pressure gradient (CPG) for massive fines detachment. Second, a mathematical model was developed to describe single particle detachment in the fracture, accounting for the coupling effects of hydrodynamic and extended-DLVO forces. Effects of fines size and fracture aperture on fines detachment were analyzed, and CPGs were determined from the proposed model. Modeling results revealed that the pressure gradient required for fines detachment first decreased with increasing fines size, reached a minimum value, and then increased; these minimum values are defined as CPGs, which exhibit a strong negative correlation with fracture aperture. CPGs obtained from modeling were slightly smaller than those determined from experiments, due to the assumptions of homogeneous surfaces and spherical particles in the model. Finally, the implications of this research on field-scale fines control in coal were thoroughly discussed.