The descriptions of the paramagnetic-ferromagnetic and paraelectric-ferroelectric phase transitions (PTs) by Weiss's molecular field theory (WMFT) are quite successful, and the WMFT is also a theoretical basis of initial analysis of the PTs of structural disorder and complex compositional systems because of its mean-field characteristic. However, there is not any study on the PT behaviors of the WMFT with external field, and only the case of two orientational states of molecules for ferroelectric systems has been investigated by the WMFT. Although the descriptions of the above two kinds of PTs by the WMFT are quite similar, the exact ones and the corresponding results are different more or less due to the difference in microscopic unit between the magnetization and polarization In this work, the exact descriptions of the ferroelectric systems with arbitrary orientational states of molecules by the WMFT are provided, including the temperature dependences of the spontaneous polarization, the internal energy, the specific heat and the static polarizability, and then the evolutions of the PTs with an external field are studied. The obtained results are as follows. 1) Without the external field, the PTs of the systems are of the second order, and the transition temperatures and spontaneous polarizations decrease, which are different from those of the ferromagnetic systems, but the internal energy, the specific heat and the polarizability increase with the increase of the orientational states. 2) The external field drives the second order PT to a diffusive one, and diffusive temperature range becomes wider as the field is increases. The results mentioned above would benefit the deep studies of ferroelectric PT, especially the diffusive one.