The deceleration of matter in a radioactive shock wave has been mathematically simulated. This problem is relevant to the variable radiancy of T Tauri stars. We assume that an ordered magnetic field at the surface of a star is not too great, i.e., B ≤ 100 G. In this case, the initial deceleration and plasma heating (at the entrance to the cooling zone) occur in a fast magnetohydrodynamic (MHD) shock wave. To calculate the intensity of radiation losses, we use real and power radiative functions. The stability/instability of a radiative shock wave is considered as a function of the incoming flow’s parameters: velocity, strength of the magnetic field, and its inclination to the surface of the star. In a series of calculations with the real radiative function, a simple criterion for stability at an accretion velocity of 1.3 × 107 cm/s has been found. The results of modeling with the real and power radiative functions are compared.