Tungsten is a candidate material for the first wall and divertor in a tokamak fusion reactor, in which it is required to withstand a high neutron irradiation. The defects created in cascade form the primary state of damage and their subsequent evolution gives rise to important changes in their microstructures and engineering properties. However, the evolution and aggregation of radiation-induced defects in atomic level can not be observed by experiments up till now. In this work, molecular dynamics (MD) method was used to explore the microstructural processes and atomic mechanism of the formation and evolution of defects in the initial stage of radiation in bcc-W. The range of primary knock-on atom (PKA) energies is 1.0~25.0 keV, and simulation temperature range from 100 to 900 K. The number and distribution of defects produced by displacement cascades have been studied; the influence of PKA direction and temperature on the number of steady Frenkel pairs has also been researched; defect clusters and the threshold energy have been simulated. The results showed that for morphology distribution of defects induced in the peak time of cascade, the more intensive the defects are, the less the steady Frenkel pairs numbers are, on the contrary, the more decentralized the defects are, the more the steady Frenkel pairs numbers are; the number of steady Frenkel pairs is insensitive to PKA direction, but has a trend to decline with the temperature elevating; the percentage of interstitial clusters is higher than that of the vacancy clusters, while vacancies tend to form larger clusters; the average threshold energy of W is less affected by temperature and has certain anisotropy. The results of this work can provide data for analyzing the behavior of W material under nuclear environment.