The paper describes the motion modeling of a fire helicopter with an external sling and a spillway device. The mathematical model of the fire helicopter with an external sling takes into account the effect of the stochastic disturbances for the sling caused by the rotor vibrations, gusts of wind and convection flows in the fire zone. The stochastic optimal control is synthesized with minimal sling oscillations and control power for its implementation. The solution is obtained due to dynamic programming. The resulting optimal control does not depend on the mass of a spillway device, but it is a function of a sling length. Thus, because of the changing sling length during motion, the length should be measured together with the sling deviation angle and the intensity of the stochastic disturbances. The temporal evolution of the system solution is done for various disturbance intensities. The computer calculation results are presented as a function of the suspension length and the intensity of the stochastic disturbances. Analysis of the results shows that the sling length affects significantly the value of the control signal, especially at high levels of the disturbances. A structure of the helicopter stochastic control system is proposed. It makes it possible to control oscillation of the external sling within an acceptable range. The system includes an oscillation angle, a sensor of the stochastic disturbance intensity and the sling length sensor. Thus, the helicopter control can be obtained through changing the thrust magnitude of the main and tail helicopter rotors.