The potential mechanism responsible for water-weakening effects on the direct tensile failure of rocks remains poorly understood due to the difficulty of experimental control and the occurrence of multiple water-rock interactions. To obtain such knowledge, a series of direct tension tests of marble rock under dry, natural and saturated conditions, coupling with the acoustic emission (AE) signals monitoring, were conducted. Some indices, such as AE energy, the ratio of rise time to amplitude (RA) and the average frequency (AF), were unitized to investigate the micro failure process of marble rock. Test results show that the tensile strength of marble rock significantly decreases with the increase of water content, characterizing variable fracture surfaces. The cumulative AE energy released in each loading stage (i.e., entire loading stage, loading stage before peak stress, and loading stage after peak stress) significantly decreases with growing water content. The greater reduction rate of cumulative AE energy released before peak stress can be result from prominent frictional weakening effect induced by water. Also, investigation of crack classifications and failure patterns based on the polarity and AE parameter methods suggests that, the number of tensile cracks, which dominates the progressive failure process of rock under direct tension, is positively correlated to the water content. The crack classification criterion described as the optimal ratios of AF and RA values in AE parameter method is found to be affected by water content. Finally, the primary mechanism for the marble rock deterioration under direct tension is friction weakening, and the effect of pore water pressure is very limited due to the loading regime. The friction weakening effect is largely attributed to the water film but not likely to the mineral dissolution.