Fluid queue model has become a powerful tool for service system modeling and analysis. In the real-time service system, customers may have optional additional services in addition to the necessary basic services. On the other hand, the service system may operate at low speeds due to various factors such as performing other tasks, which reduce the service efficiency. Taking the above situation into consideration, a new fluid queue model is constructed based on a multi-server queuing system with optional services and working vacation as an external random environment. The steady-state queue length distribution of driving system is obtained by means of quasi birth-and-death process and matrix-geometric solution method, and the net input rate structure and steady state conditions of the fluid model are given. The matrix differential equation satisfied by the steady-state joint distribution of fluid model is derived by using probability analysis method and stochastic process theory. Several important function sequences are constructed according to the structural characteristics of the above matrix differential equation. With the Laplace transform (LT) and Laplace-Stieltjes transform (LST) methods, the steady-state distribution of fluid level in the buffer, mean fluid level and the probability of empty buffer are given. Finally, numerical analysis is presented to analyze the influence of system parameters on performance indices. The results show that the system performance can be optimized and the service efficiency can be improved by adjusting the model parameters in practical background.