For application of strontium isotope to study of paleokarst,the paper has systematically analyzed the calcites filled in fissure-cave of the Ordovician carbonate rocks in the Tahe oilfield,Tarim Basin.The research results indicated that lower 87Sr/86Sr ratios are a character of calcites formed in the karst during the Middle Caledonian,and the 87Sr/86Sr ratios of calcites are controlled by those of host rocks.However,the calcites of Early Herlynian paleokarst have higher 87Sr/86Sr ratios,which are controlled by the terrestrial strontium and the dissolved strontium from the wall rocks.The 87Sr/86Sr ratios of different attitude calcites differ;the products of vadose zone are enriched in 87Sr than phreatic zone,which mainly result from the seepage mechanism of the karstic system and water-rock interaction.Furthermore,the area covered with O3s Formation has higher 87Sr/86Sr ratios than the north area,and mainly contributes to an increased proportion of strontium isotope of the host rocks.Regional variations of strontium isotope and geologic backgrounds may indicate that the Early Hercynian paleokarst has important influences on the Ordovician carbonate rocks in the north area and the areas comprising the O3s Formation,and it is the main formation episode of karstic fissure-cave reservoir in the Tahe oilfield.In addition,the extremely low 87Sr/86Sr ratios may be an indicator of mantle source,and the volcanism of Early Permian is a main period of providing mantle-derived strontium.The study on the Ordovician paleokarst in the Tahe oilfield suggests that strontium isotope is of a better ability in indicating fluid's provenance,and therefore,it has distinct advantages in the division of paleokarst stages.