diffuse light has been blocked, but the ambient light still has

in reality, it should be almost black, how can i do that

If there is ambient light, it will be applied. Turn off ambient. If ambient is already off, check your normals. If normals are ok, check normal and light transform.

yeah, it's the problem of ambient

if i want to keep the ambient part of light and material, can the front triangle be black?

No. Ambient is a pure +.

If ambient is (25, 32, 48), those numbers will be added to all shaded pixels with no distance or orientation attenuation. See Mathematics of lighting. Note

Global Illumination = Ambient Light + Diffuse Light + Specular Light + Emissive Light

Also see Ambient Lighting. Ambient light is "constant" light.

thanks a lot