Internal solitary waves (ISWs) are the nonlinear large amplitude waves existing in the oceanic pycnocline. Normally, it is difficult to obtain the maximum horizontal current speed induced by ISW since most of the passage periods of ISWs are no more than 30min while the observational sampling interval is too low; meanwhile, to avoid power shortage of the mooring Acoustic Doppler Current Profiler-carried electric battery, oceanographers who are only concerned about the study of circulation or internal tide would choose a sampling interval of no less than 5min. Can we use these low-time-resolution observational data to retrieve the maximum horizontal current speed induced by ISW? In this article, based on Korteweg-de Vries (KdV) equation theory (shallow-water theory), we propose a method to retrieve the maximum horizontal current speed induced by ISW. According to the calculated results based on the observational current data of ISWs near the Dongsha Islands in the northern South China Sea, it is found that, our method could well retrieve the maximum horizontal current speed induced by ISW from the low-time-resolution observational current data series, meanwhile, when the sampling interval of observational data or the ISW characteristic half-width increases, or when the ISW nonlinear phase speed decreases, the relative error of the retrieved maximum horizontal current speed will reduce. The applicability of the method to ISWs in finite-depth is also discussed.