Thermoacoustic analysis of an open Helmholtz photoacoustic cell was performed using linearized continuity, momentum, energy and state equations. To mimic the open boundary of the cell, an external pressure acoustic domain encapsulating the cell was considered with radiation boundary condition at the outer surface and coupling of the thermoacoustic and pressure acoustic domains was done at the open boundary of the cell. Effect of the diameter of the opening of the cell on the acoustic pressure, quality factor and the resonance frequency were analyzed. The possibility of improving the cell performance by extension of the opening with an additional fixture was also studied. Our result shows that while the radiation losses through the opening is not significant at smaller diameter of opening, for larger opening, extension of the opening by additional fixture helps to improve the cell performance.