Abstract: Fast-spectrum micro-reactor has many technical advantages, such as high energy density, good mobility, fast deployment speed, long life and so on. Because of the characteristic of compact structure and high role of leakages in the neutron balance in fast-spectrum micro-reactor core, geometric deformation caused by temperature change and mechanical effect under external force is the main source of reactivity feedback. So it is very important to carry out transient analysis based on neutronics-thermo-mechanics coupling to accurately simulate the dynamic characteristics of fast-spectrum micro-reactor core. Based on the open source multi-physics coupling framework MOOSE, a three-dimensional coupled neutronics thermo-mechanics model for the dynamics analysis of fast-spectrum micro-reactors was developed in this paper. The thermo-mechanics coupling was solved by using the JFNK (Jacobian-Free Newton Krylov) method. Then the neutronics model was coupled with thermal-mechanics model by Picard iteration method through the MultiApp and Transfer system based on MOOSE framework. The neutronics solver can directly simulate the neutronics behavior of deformed nuclear reactor configurations including automatic adapting of the cross sections for account for density change. In this paper, this method was applied to the simulation of the supercritical transient of Goidva-Ⅰ with three-dimensional coupled neutronics thermo-mechanics model. Time evolution of fission rate, average temperature rise, surface displacement and surface velocity in the transient with an initial period of 16.2 μs were evaluated. The numerical result of fission rate during the transient was compared with the experimental data, and an overall good agreement was observed. From the numerical results, it can be seen that the fission rate increases, then reaches a maximum value with the positive reactivity introduced. Goidva-Ⅰ is a typical compact, fast-spectrum reactors, reactivity feedback is dominated by core deformation. The quickly accumulated thermal power causes the core temperature increasing, the reactor materials expand, resulting in a larger geometry and reduced material densities. Fission rate then drops due to loss of criticality caused by the thermal expansion. Due to the rapid increase and decrease of fission power, the core is in a state of compression and expansion one after another, and the thermal inertia effect leads to the oscillation of core surface displacement and velocity. At the same time, due to the change of core reactivity caused by deformation, the core power also produces periodic oscillations after 500 μs. The results show that this method can accurately consider the reactivity feedback effect caused by the thermal expansion, which lays a foundation for further multi-physics coupling safety analysis of fastspectrum micro reactors.