In this paper, we regard the smooth quadric threefold Q3 as Lagrangian Grassmannian and search for fixed rational curves of low degree in Q3 with respect to a torus action, which is the maximal subgroup of the special linear group SL(2, C). Most of them are confirmations of very well-known facts. If the degree of a rational curve is 3, it is confirmed using the Lagrangian’s geometric properties that the moduli space of twisted cubic curves in Q3 has a specific projective bundle structure. From this, we can immediately obtain the cohomology ring of the moduli space.
MSC Classification: 14E05 , 14E15 , 14M15