Sodium transition-metal phosphate NaMPO4 is known to mainly present two polymorphs: the thermodynamically metastable and electrochemically active olivine and the thermodynamically stable and electrochemically inactive maricite. The direct synthesis of the olivine and the activation of the maricite thus become the focuses of preparation of high-performance phosphate cathode materials for Na-ion batteries. To obtain better understanding of these two issues, the structural stabilities and the ionic transport properties of NaMPO4 polymorphs are investigated using atomistic simulation method. Free energy calculations predict pressure-induced structural phase transitions from maricite to olivine at low or negative pressures, and the critical pressures shift towards the negative pressure region as the applied temperature increases. It suggests that the introduction of negative pressure or lattice expansion will increase the structural stability and accordingly facilitate the direct synthesis of olivine NaMPO4. Migration energy calculations reveal a less direction-dependent ion migration for maricite lattice, which may contribute to the activation of nano-sized maricite NaMPO4. Interestingly, the calculated energy barriers decrease with the increase of the ionic radius of M2+ ion. Chemical substitution with larger divalent cations to increase the average ionic size of M2+ ions is thus expected to further improve the rate capability of maricite Na-phosphate.