We present a calculation of quantized Landau magnetic levels in thin films, using a quasi-classical approximation. It is shown that a symmetric triangular potential barrier is formed inside the film by applying low magnetic fields. Electron tunneling through this potential barrier results in splitting of the magnetic surface levels, for which a simple expression is obtained and a numerical example is presented. It is shown that in thin films the upper quantum states are limited by the film thickness. The details of the calculations and their implications are discussed.