Inlet unstart boundary is one of the most important issues of the hypersonic inlet and is also the foundation of the protection control of scramjet. To solve this problem, the 2-D internal steady flow of a hypersonic inlet was numerically simulated in different freestream conditions and backpressures with a RANS (Reynolds-Averaged Navier-Stokes) solver using a RNG (Renormalization Group) k-ε turbulence model. The dimensional analysis method was introduced to find the variables describing the inlet unstart boundary based on "numerical experimental" data in this paper. The dimensionless pressure ratios of forebody and isolator were analyzed respectively. The results show that the unstart boundary of the hypersonic inlet is determined by M0, α and Re0. Pressure ratio π increases with M0 increasing, and it increases first and decreases then with α increasing. M2 decreases with α increasing. The low Mach number unstart phenomena occurs when M2 is less than the Ms2. Pressure ratio π increases with Re0 increasing. Re0 (Re0<2×107) has a major effect on n and Re0 (Re0>2×107) has little effect on π.