Perovskites have been recognized as a class of promising materials for optoelectronic devices. We intentionally include excessive Cs cations in precursors in the synthesis of perovskite CsPbBr nanocrystals and investigate how the Cs cations influence the lattice strain in these perovskite nanocrystals. Upon light illumination, the lattice strain due to the addition of alkali metal Cs cations can be compensated by light-induced lattice expansion. When the Cs cation in precursors is about 10% excessive, the electron-phonon coupling strength can be reduced by about 70%, and the carrier cooling can be slowed down about 3.5 times in lead halide perovskite CsPbBr nanocrystals. This work reveals a new understanding of the role of Cs cations, which take the A-site in ABX perovskite and provide a new way to improve the performance of perovskites and their practical devices further.