<p>Gas breakdown induced by focused terahertz beams exhibits novel characteristics and bright application prospects, making research in this field of great significance. In this paper, numerical simulations of argon gas breakdown induced by a focused terahertz beam are carried out using a plasma fluid model. In this model, rate coefficients such as ionization rate are determined via the Boltzmann equation solver BOLSIG+. An effective electron diffusion coefficient is employed to describe the transition process from free diffusion to ambipolar diffusion. The fluid model under axisymmetric conditions is solved using the finite difference method. Simulation results show that the ionization rate derived from BOLSIG+ is in good agreement with the results of the particle-in-cell-Monte Carlo collision model. At the same mean electron energy, the ratio of ionization rate to gas density varies with pressure. When the peak electric field of the focused beam reaches the breakdown threshold of long pulses, radial electron diffusion still exerts an influence on the breakdown process, even under conditions of high gas pressure and uniform distribution of seed electrons. This is because the 1/<i>e</i> spot radius of the focused beam is small (on the order of millimeters), and the plasma density generated by ionization has a large gradient. As the pressure decreases, the influence of radial electron diffusion on the breakdown process becomes more significant. The accuracy of the fluid model is verified by comparing the simulated values of the breakdown threshold of the focused beam with the experimental results.</p>