First,the multiple solitary waves solution of a variable coefficients KdV equation with forcing term has been obtained by using the improved homogeneous balance method. Second,the multiple solitary waves solution of the forced KdV equation has been obtained with the help of above obtained solution. At last,as an example,the interactions of Rossby solitary waves were analyzed by using graphics analysis method, and the main reasons which influence the relative amplitude,phase,travelling direction and equilibrium position of Rossby solitary waves were pointed out.