The evolvement of defocus aberration in unstable resonator is studied in geometric optical approximation. Linear correction in a positive branch cavity is obtained analytically. The simulation results from oxygen iodine chemical laser show that the smaller the magnification of the resonator is, the greater the influence of aberration evolvement becomes. Deformable mirror located at concave mirror can obtain the best compensation effect. The value of β can be reduced by 35% on average with considered aberration sensitivity compensation compared to the direct compensation.