Abstract: In this paper,the change of the Rossby parameter β with latitude is considered and the parameter γ≡dβ/dy=2sinφ/a² is introduced and the β-plane approximation is extended into f=f₀+β₀y-γ₀y²/2 which includes the parameter γ.Such approximation closes further to practice especially in the high latitude regions.We give emphasis to the research of the effect of the parameter γ on the Rossby waves.It is seen that the effect of the parameter γ is remarkable in the high latitude regions.It can produce the Rossby waves caused by the pure parameter γ.And the phase speed formula of Rossby waves with the change of ft is generally given,which is degenerated into the well-known Rossby formula when γ₀=0.The researches also point out that when the change of β is regarded,even if the basic current u is a linear function of y the unstable modes can also take place.However,the parameter γ usually plays a stable part in the Rossby waves and it does affect the longitudinal scale and the structure of constant phase lines(trough-ridge lines) of Rossby waves and slow down the growing or decaying of Rossby waves.