Investigation into effects of conformation variation on optical absorption of phycobilins is important in photosynthesis research. It is found that the electronic excited-state of phycobilin exhibits a quasi-Boltzmann distribution due to inhomogeneous random variation of its con formation, and that the envelope of the vibronic absorption transition band can be described as convolution with the inhomogeneous random distribution of con formation variation. Inhomogeneous random distribution of phycobilin conformation variation results in asymmetric broadening of vibronic absorption transition band.