The present disclosure relates to a kit and method for testing harmful heavy metals onsite, which can determine the presence or absence of harmful heavy metals and the degree of contamination with harmful heavy metals dissolved in water or contained in sediment, soil, deep-sea mining tailings and the like. The kit includes a receiving member and a diagnostic solution containing a biosurfactant, which is accommodated in the receiving member and undergoes an ion exchange reaction with a liquid including harmful heavy metals or a solid sample from which harmful heavy metals are leached in a liquid phase.