Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose 20 kton liquid scintillator detector being constructed in southern China. It aims to measure the neutrino mass ordering using reactor neutrinos. The key to this measurement is the superior energy resolution of JUNO, which is directly affected by the waveform reconstruction of the 17612 20-inch PMTs to be installed on the detector. This poster presents a data-driven approach for the waveform reconstruction based on calibration, motivated by the characteristic long-tail of the single photoelectron charge spectrum of the MCP-PMTs used in JUNO and their effect in the energy resolution.