Based on rigidbody modeling slungload hypothesis, a nonlinear dynamics model of the helicopter with slungload coupl i ng system is presented, and the rigidbody modeling slungload brings in ex tr a degrees of freedom and constraints, which makes the nonlinear equations of helic o pter motion increase from 9 orders to 19 orders. The nonlinear equations are li n earized by small perturbation hypothesis and trimmed by continuation. Then the t rimmed results are compared with the experimental data of helicopter flight and the cal culation results of helicopter with masspoint modeling slungload. The motion modes of helicopter with rigidbody modeling slung  load hypothesis are calculated with difference method, and the impacton stabili ty of helicopter by extra slungload is analyzed at the same time. Results sho w that the rigidslungload adds five new motion modes to the system, two of wh i ch about sling swinging are stable, three of which about slungload attitude mot i on are unstable. Meanwhile, the introduction of slungload makes the sh o r t period mode and the roll mode of helicopter unstable, which has a strong impac t on the flight quality of helicopter.