Disclosed are a high-temperature-resistant tertiary amine-grafted silica nano-plugging agent and water-based drilling fluids, relating to oil-gas field drilling. The nano-plugging agent is prepared from a hydrazine compound, a diallyl compound, an enamine compound, silica particle, and an amino-containing silane coupling agent by stepwise reaction. The nano-plugging agent has a grain size of 58-280 nm.