The building I want to design three-dimensional is complicated process- my question
is that What rule do i write for this building to Realistic mode of designed
When designing complex geometry, you have to decide if you can approximate it with pieces of primitives (cubes, spheres, cylinders, cones), or if it's just too complicated to make it looks good by combining a bunch of primitives or parts of primitives.
In this case, since the geometry is complicated (i.e. I don't see an easy way to approximate it with primitives), I would recommend making the curvy roof part of the building in a 3D modeling software such as Maya, for example, and exporting it as an obj file, for example. Then, you could insert the asset in your roof rule using the i() operation.
ESRI provides Tutorials for CityEngine. In Tutorial 7: Facade modeling—CityEngine Tutorials | ArcGIS Desktop. With it the users can practice modelling more complex and detailed buildings with CGA. It is shown how to instance pre-modeled facade elements like arcs.
CityEngine supports import of many commonly used 3D formats, this allows to create the assets with a wide variety of modelling tools. I am confident you will be able to come to a satisfying result.