The invention relates to a testing device for simulating a seepage failure of a dam with upper part defects and a construction method thereof. The testing device comprises a model tank and a dam model arranged in the model tank. An arch simulative ant channel is formed in the upper part of the dam model. A spherical area is arranged near the position, close to the downstream dam slope, of the simulative ant channel of the dam model, and an artificial sprinkler is arranged above the model tank. A plurality of sets of monitoring sensors are buried near the simulative ant channel in the dam model. The construction method of the testing device comprises the steps that firstly, clay is made into a soil sample, an arch thin pipe is buried on the upper part of the dam model, and tamping is conducted after the soil sample is made higher than the thin pipe through filling; and secondly, the portion, on the periphery of the thin pipe, of the soil sample is excavated to be spherical, the excavated portion of the soil sample is backfilled again to make a clay layer in the spherical area become looser towards the thin pipe, the simulative ant channel is generated by pulling out the pre-buried thin pipe after filling is completed, and the multiple sets of monitoring sensors are arranged along the simulative ant channel. The testing device can be used for studying preferential flow effects of the upper part defects of the homogeneous dam and the disaster-causing mechanism inducing the seepage failure of the homogeneous dam.