Sodium is proved to be able to improve the performance of Cu(In,Ga)Se2 solar cell grown on flexible polyimide substrate by a low-temperature deposition. Different sodium incorporation methods affect the film in different ways. Comparing the deposition of a NaF precursor with post deposition NaF treatment, different mechanisms can be found. In NaF precursor approach, Na is available during the Cu(In,Ga)Se2 growth and acts as a surfactant at the grain boundary which adds up an energy barrier for adatom to across. Thus, a small grain size as well as double-peak reflection pattern can be observed and the formation probability of donor defects is reduced. In post deposition NaF treatment, incorporation of NaF does not affect the growth and microstructure but passivates donor defects in the Cu(In,Ga)Se2 film. Moreover, according to the experimental results, external energy assistance is necessary during NaF incorporation through post deposition treatment. It is verified that Na incorporation is able to improve the properties of the film effectively when substrate temperature reaches above 350 ℃. Finally, the conversion efficiency of flexible Cu(In,Ga)Se2 thin film solar cell on polyimide substrate is achieved to be 10.4% by optimizing the sodium incorporation.