Abstarct: Lasjard spring is a karst spring located 10 kilometers west of Sorkheh in Semnan province. The most important geological formations of the region are the karaj formation and qom. In this research, in order to know the water resources involved in the feeding of Lasjard spring, conventional methods such as hydrograph analysis of Lasjard spring, investigating the time series behavior of its physicochemical parameters were used. It was also tried to use the data of stable isotopes of oxygen 18 and deuterium to obtain important information about the sources of power of the mentioned spring. In order to determine the hydrogeological characteristics of Lasjard spring and its hydrochemical and isotopic characteristics, sampling of the spring and nearby water resources was done in the water year 97-98. Examining the results of the recssion curve and the low values of recssion curve coefficients of Lasjard spring showed that the spring has a descent regime which can indicate the long path of the outflow from the spring. The dominant type of the samples is calcite sulfate, but the main ions of the samples have different concentrations, which show that their origin is different. The isotopic samples in all the samples were different, except for the isotopic samples of the Lasjard spring and the isotopic sample of the precipitation at higher altitudes of the basin, where these two samples were in good agreement with each other. With the field visit and the geological and structural geology (tectonic) evidence of the region, it shows that the existence of a fault that comes from the limestone heights of the Qom formation located in the southwest of Jovin, plays an important role in the water supply of Lasjard spring and this area has the potential to form the catchment area of this spring. Therefore, the catchment area of this spring was identified at these heights and the preliminary catchment area was drawn. Due to the lack of a reliable source about the percentage of nutrition in the rocks of this area, using parameters that affect the amount of infiltration, such as the intensity, amount and type of precipitation, lithology of the area, karst effects, blocked pits and The sunken areas, the seams system and the direction of the slope are prepared by Arc Gis, the desired information laxyers and it has been used to obtain the percentage of feeding of the catchment basin baxsed on that The feeding percentage of the target area was calculated as 46% and the area of the primary catchment area was 34 square kilometers, then the identified primary catchment area was evaluated using the hydrogeological balance method.