The phase transitions of molecular system consisted by double rectangular plates connected by flexible chain are studied in this paper. Based on a simple interaction energy between molecules and the approximation of mean-field theory

as the interaction strength of flexible chain increases

four kinds of phase diagrams in the plane of molecular structure and temperature are obtained. In the first kind (where the interaction strength is the smallest) there is only one Landau point that allows system directly transit from isotropic phase to biaxial phase. In the second kind

Landau point spreads to a Landau curve. The region of positive uniaxial phase increases and that of negative uniaxial phase decreases. In the phase diagram

a first order transition from uniaxial phase to biaxial phase appears. In the third kind Landau point also spreads to a Landau curve

but the region of negative uniaxial phase disappears. In the fourth kind(the interaction strength of flexible chain is the largest)

the range of Landau curve becomes narrower.