The invention provides modified bentonite for seawater drilling and a preparation method thereof, and a drilling fluid. The modified bentonite for seawater drilling comprises the following components:bentonite, sepiolite, palygorskite, kaolinite, illite, chlorite, opal, calcite, siderite, talcum powder, gypsum, pyrite, hornblende, quartz, nanometer iron oxide particles, nanometer silicon dioxideparticles and nanometer graphite. The bentonite prepared from the components is high in slurrying speed, low in sand content, good in slurry making rate, high in sand carrying capacity, low in filterloss and high in stability.