The way of development of modern space optical sensors is large aperture and long focus length. Long focus length means the distance between primary and secondary mirror will be longer, and tolerance of secondary mirror means more difficult to guarantee. For the secondary mirror strict tolerances, a design of space truss structure is presented. According to the theory of statically determinate spatial frameworks, two schemes are designed and were analyzed through finite element analysis software. We got the first three inherent frequencies, the angle around the X axis and the rigid body displacement along Y axis. The results show that the first mode of the scheme 1 is 78 Hz, the angle around the X axis is 2″, and the largest rigid body displacement along the Y axis is 0.04 mm. For scheme 2, the first mode is 113.7 Hz, the angle around the X axis is 1.5″, and the largest rigid body displacement along the Y axis is 0.022 mm. Two schemes can meet the design requirements, while scheme 2 is more excellent than scheme 1. The theory of statically determinate spatial frameworks provides a reliable theoretical basis for the structural design of space camera.