Abstract: To reveal the meso-mechanism of crack coalescence, this paper uses particle flow code(PFC) to simulate uniaxial compression test of gypsum specimens containing double parallel pre-existing flaws. The paper classifies the crack coalescence types in the specimens with different geometry of double pre-existing flaws, and analyzes the force chain and particle displacement field around the pre-existing flaws. It is found that the relative position of pre-existing flaws can affect the contact force distribution and particle displacement filed around the pre-existing flaws, and then affect the crack propagation path. When the two pre-existing flaws are nearly coplanar, there is a concentration of contact force in the rock bridge area. Micro-tensile cracks first appear in the rock bridge area and gradually evolve into macro-shear band with the loading. These micro-tensile cracks lead to type Ⅰ coalescence. When the rock bridge inclination angle is large and the two pre-existing flaws do not overlap, the contact force concentration in the rock bridge area increases. Vertical micro-tensile cracks first appear in the rock bridge area. Macro-cracks evolving from micro-tensile cracks connect pre-existing flaw tips, resulting in the type Ⅱ or Ⅶb coalescence. When the tips of two pre-existing flaws are partially overlapped, the compressive contact force concentration at the overlapped tips is smaller than that at the non overlapped tips due to the stress shielding effect, which leads to the wing cracks becoming the main cracks leading to coalescence. When the two pre-existing flaws overlap in the vertical direction, the contact force between the two pre-existing flaws on the same side tips is concentrated, resulting in tensile cracks caused by compression. Tensile cracks induced by compression connect the tips of the pre-existing flaws on the same side, forming type Ⅴ coalescence. The results show that the number of micro-cracks in the samples with type Ⅱ, Ⅴ and Ⅶb coalescence increases step by step, which indicates that the propagation of macro-cracks in these three coalescence types are caused by the sudden release of strain energy.