The invention relates to the field of petrochemical drilling, and discloses an environment-friendly water-based drilling fluid suitable for a shale gas horizontal well. The drilling fluid contains a nano plugging agent, a bionic wall fixing agent, a bionic shale inhibitor and a filling agent; the nano plugging agent is modified silicon dioxide nano-particles; the modified groups of the modified silicon dioxide nano-particles comprise modified acrylic copolymer chains; the bionic wall fixing agent is carboxymethyl chitosan of which the main chain is grafted with a group which is derived from dopamine; the bionic shale inhibitor consists of structural units of arginine and structural units of lysine; the filling agent consists of 1,600 to 2,500-mesh calcium carbonate, 1,050 to 1,500-mesh calcium carbonate and 500 to 1,000-mesh calcium carbonate in weight ratio of 1 to (0.55-6) to (0.55-6). According to the drilling fluid disclosed by the invention, high temperature resistance, high plugging property, high inhibiting property and high environment friendliness can be achieved, and particularly high density can be achieved, so that the drilling fluid is suitable for shale gas mining.