Temperature dependence of the upper critical field H c2 of the Dirac semimetal (DSM) with phonon-mediated pairing is considered within the semiclassical approximation. The lowerature dependence deviates from conventional BCS superconductor with parabolic dispersion relation (Werthamer N. R., Helfand E. and Hohenberg P. C., Phys. Rev., 147 (1966) 295; Helfand E. and Werthamer N. R., Phys. Rev. Lett., 13 (1964) 686) even for large adiabaticity parameter, , where μ is the chemical potential and Ω the Debye frequency. In particular the "reduced field", ratio of zeroerature H c2 to the derivative at critical temperature, , depends on γ and can be extended beyond the adiabatic limit. The reduced magnetic field ratio is universal (independent of the chemical potential, interaction strength, etc.) and smaller than the Werthamer ratio for clean superconductors: for DSM, for parabolic band. The results are in good agreement compared with recent experiments on TaP.