This study is to investigate the mechanisms of fracture extending in coal rock by pulse hydraulic fracturing.Experiments are performed on coal rock samples using a pulse hydraulic fracturing experimental system under triaxial loading by changing the pulse frequency and viscosity of fracturing fluid.Experimental results show that the fracturing response of pulse pressure and acoustic emission（AE） indicates the process of fracture extending induced by pulse hydraulic fracturing can be divided into four periods

i.e.

scattered initiation period

uniform growth period

sudden coalescence period and failure termination period.The average rates of the spatial distribution of AE locations at the uniform growth period and sudden coalescence period are 4.6 and 9.6times higher than the scattered initiation period respectively.The general tendency of i-value curves demonstrates that microcracks mainly propagate at the uniform growth period and large scale cracks primarily propagate at the sudden coalescence period.The partial b-value curves present the characteristics by combining the shape of ridge line with ladder shape.Before the fracturing pulse frequency reaches the optimum state

the duration of uniform growth period and sudden coalescence period affected by high frequency pulse pressure is shorter than that by low frequency one.Under the same frequency

the duration of uniform growth period exhibits a shortening trend

as the viscosity of fracturing fluid increases

but the duration of sudden coalescence period shows the opposite way.