This paper aims at decreasing the assembly difficulty and reducing the cost of optical systems using multiple components. A strategy is proposed to substitute one freeform surface for multi-separated surfaces. A freeform-objective Chernin multipass cell is implemented as a typical instance for assembly simplification. The application of a freeform surface helps in decreasing the degree of objective assembly tolerances from 24 to 4, by representing the original four objectives with a linear combination of a 2D Gaussian radial basis function, which is analytical, global C∞ smooth, and locally influencing. The simulation result verifies the feasibility and performance of the designed freeform-objective Chernin multipass cell, whose base length is 0.2 m, volume is less than 0.8 L, and adjustable optical length is 0.4-34 m. Results prove that by using a proper approach to approximate the original surfaces as an integrated freeform, one can simultaneously reduce the assembly cost and the assembly difficulty.