The present invention discloses an ultra-high performance concrete with compressive strength of 300MPa or more and a preparation method, and the ultra-high performance concrete comprises the following components in parts by mass: 100 parts of cement, 15 to 70 parts of an admixture, 1 to 5 parts of a nanomaterial, 1 to 20 parts of a viscosity reducer, 110 to 290 parts of sand, 20-50 parts of fibers, 2 to 5 parts of a superplasticizer, 0.1 to 0.5 part of a powder dispersing agent and 15 to 25 parts of water. The ultra-high performance concrete has ultrahigh mechanical properties, the compressive strength after heat curing at 85 DEG C-90 DEG C in standard atmospheric pressure environment can be 300MPa or more, the flexural strength is greater than 55MPa, and the ultra-high performance concrete has excellent working performance and ultrahigh durability.