Three-wave coupling process in parametric excitation is further studied under an isotropic,homogenous and collisional ionospheric background.A general dispersive equation is firstly derived to describe the excitation of waves for a collisional ionosphere.Then,to start with this dispersive equation as well as the two kinds of wave frequencies and vectors matching conditions obtained in the paper [1],the threshold fields of pump waves to trigger overdense and underdense parametric instabilities and the growing rates of the excited plasma waves are obtained and compared with each other.The results show that in the case of overdense heating the threshold value is proportional to both ion and electron collisional frequency,which is much different from the previous results that the threshold only holds a proportional relation with electron collisional frequency.It is also concluded that more powerful HF waves are required to give rise to parametric instability for underdense heating than overdense heating.