Using the general unitarity cuts method and amplitudes approach, we calculate the 4-point all-plus-helicity graviton amplitudes at 2-loop. This reproduces a well-known result about 2-loop divergence in quantum gravity, and more importantly, we figure out a very simple renormalization scale dependence of gravity theories at 2-loop. And from this scale dependence, we conclude the duality between scalar and 2-form, between 3-form and cosmological constant at quantum level. And after this direct but complicated calculation, we figure out an alternative simple derivation by doing the cuts and integration in 4d, instead of 4-2e dimension. This elucidates the ultraviolet(UV) physics within. Besides, using techniques from amplitudes, we calculate the bending angle of massless projectiles, including graviton, when they pass near a massive object, like the sun, which is represented by a massive scalar. This reveals the long-distance/infrared(IR) properties of quantum gravity, without worrying about the UV details. And we obtain different bending angles for different massless projectile with different spins, which could possibly indicate a violation of classical equivalence principle at quantum level.