The optimization problem of space transfer of the spacecraft to Phobos is considered. On the one hand this problem is coincided to real mission to Phobos, which Russian Federation is going to realize in the next few years. On the other hand, the method development of the spacecraft interplanetary trajectory optimization is a question of present interest. The authors of researches usually neglect the planetocentric legs of trajectory in such problems, and there is no through optimization of the whole mission. In this paper the method of Pontryagin's extremals design in such problems with the taking into account these features by the example of flight to Phobos. The coordinates and velocity vectors of the Earth, Mars and Phobos correspond to ephemeris DE424 and MAR097. Gravitational fields of the Sun, the Earth and Mars are considered to be Newtonian. The spacecraft is equipped with High- and Low-thrust engines. The problem of cosmodynamics is formalized as an optimal control task and then is solved numerically by shooting method. In this paper certain trajectories are designed, and the possible gain due to using of combined propulsion in comparison with using only High-thrust engines is considered.