POLAR is a gamma-ray burst polarimeter on board TianGong2 space lab, which was launched into low earth orbit in 2016 September 15. It is designed to measure the linear polarization of hard X-ray in the 50--500keV energy range. The detector has 25 modules, and each module has 64 plastic scintillator bars. With a total number of 1600 plastic scintillator bars, POLAR has a very large effective detection area and field of view. During its in-orbit operation, POLAR detected multiple small flares. Those hard X-ray photons are usually less than 50keV and can not be analyzed directly by applying in-orbit and ground high energy calibration results. With simultaneous RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) observations of a flare SOL2016112907, this paper calibrates smaller than 50keV low-energy response of POLAR with Monte Carlo simulations. We find that energy threshold (sim 10keV) and conversion factor obtained via calibration are relatively stable, but they are different from values obtained via calibration in high-energy range, and there is no evident pattern in these differences.