The invention provides a design method for a vertical constant force system. The vertical constant force system comprises a balance point constant force spring module, a gear and rack compensation module and a data acquisition and control module and is used for compensating the gravity generated in the process of ground validation of a spacecraft. The balance point constant force spring module comprises a compressed spring, a linear guide rail, linear bearings, rolling bearings, a spring jacket and connecting parts and is used for providing constant force at a balance point. The gear and rack compensation module comprises a gear and rack set, a motor and a fixing plate. When the spacecraft deviates from the balance point, the gear and rack compensation module can drive the balance point constant force spring module arranged at the lower end of the gear and rack compensation module to do acceleration and deceleration motion or uniform motion in the vertical direction to adjust force on the compressed spring, so constant force output is guaranteed. The data acquisition and control module comprises a motor driver, a control card, a high-precision tension sensor, a displacement sensor, a data acquisition card and an upper computer PC. The vertical constant force system can be used for providing the constant force in the vertical direction and compensating the gravity of the spacecraft and has the advantages being stable in operation, not prone to external interference, high in constant force accuracy and the like.