By using the rotation viscometer, the rheological properties of fresh cement pastes with fly ash were tested. Also, the relationships between fly ash dosage, the characteristic of particle group of fly ash and the rheology of cement pastes with fly ash were discussed. The test results showed that rheological behaviour of fresh cement pastes follows Bingham model and mixing fly ash does not change its rheological property. The roundness and specific surface area of fly ash can affect yield stress, plastic viscosity and thixotropy. The thixotropy of cement pastes with fly ash follows stress thinning behaviour. Both breakdown constant B and destruction area A can show quantitatively cement paste's thixotropy.