Liquid-crystal elastomers (LCEs) are an exciting kind of smart material that can convert thermo- and opto-stimuli into mechanical actuations. In this paper, we formulate an opto-mechanical constitutive relation of soft LCEs for different director orientations, which shows that the soft material becomes transversally isotropic and quasi-soft under light illuminations and also the opto-strains are produced. We consider plane stress problems for simplification, with director variation through the depth of the beam-shaped LCEs. Detailed finite element simulations of two-dimensional free standing samples indicate that both the direction and the magnitude of the light-induced bending strongly depend on the orientation of the director and that the LCEs can behave completely soft under light illuminations at a critical orientation of the director, namely, the samples deform stress-freely in shear without bending. FE results show that large shear deformation is inevitably produced due to the rotation of the director, which means that the cross section plane will not remain normal to the middle line after deformation. And even some distortions may happen to cross sections, particularly when the orientation of LC is not parallel or perpendicular to the direction of light propagation.