Abstract: Low activation ferritic martensitic steel is considered as one of the candidate structural materials for the fourth generation reactor and fusion reactor. Low activation ferritic martensitic steel is based on Fe-Cr alloy, in which W is an important solute element. Irradiation embrittlement at low temperature is one of the important problems limiting the service of low-activation ferrite martensitic steels. The precipitated clusters during the service process cause hardening and embrittlement of the material due to the obstruction of dislocation movement by the precipitated clusters. An in-depth understanding of cluster precipitation behavior during service is helpful to understand the irradiation embrittlement of low-activation ferrite martensitic steels at low temperature. In order to simulate the precipitation kinetics of Fe-Cr alloy, by introducing composition-dependent pair potential, some improvements were made on the MIET_AKMC software developed by ourselves. The precipitation kinetics of coherent Cr-rich precipitates/clusters in Fe-Cr(8%, 10%, 16%, 20%)-W(0%, 1%, 2%) alloys during thermal ageing was simulated using the atomic kinetic Monte Carlo (AKMC) method. After thermal ageing, Cr precipitate into clusters to form Cr-rich precipitates/clusters, while W is still in a solid solution state. At the beginning of coarsening process, the radius of clusters, which was about (0.57±0.03) nm, was independent on simulation temperature and the initial Cr content in the box. The precipitation process of Cr cluster can be divided into three stages: nucleation, growth and coarsening. The Cr concentration in box and simulation temperature have an influence on the precipitation rate of Cr clusters. The presence of W can delay the precipitation kinetic process and the effect is most significant when Cr content is about 10%. This is due to that when Cr content is about 10%, the absolute value of the short-range order parameter for Fe-Cr has a maximum, quite independent of the composition and the temperature. The delay effect of W was related with the stronger binding energy between W and vacancy. For the binding energy between Cr and vacancy is 0.06 eV, while the binding energy between W and vacancy is 0.14 eV. From the simulation results, it can be deduced that W is benefit for materials performance, expect for solution strengthening, it can delay the precipitation process of Cr-rich clusters, which is harmful for materials performance.