Allelochemicals are the predominant constituents identified from root exudates, which have physiological and biochemical functions and take effect on soil biota. Root system is the important passage for allelochemicals release into environment, and hence, researches on the allelochemicals excreted from it are of academic and practical significance. This paper summarized the varieties of allelochemicals from crop root exudates and their allelopathic effects, highlighted their transportation, transformation and affecting factors, and analyzed their relationships with biotic and abiotic factors. This review also described the methodologies in studying allelochemicals from root exudates. There are many chemical methods used for the separation and identification of allelochemicals. One of the common used techniques for collecting root exudates in large amount is CRETS (continual root exudates trapping system). From the standpoint of soil ecology, the direct effects of allelochemicals from crop root exudates on soil biota (bacteria, fungi, nematode etc.) were specifically discussed, and the key function of allelochemicals from root exudates in soil was presented. As allelochemicals from plant root exudates are becoming a hotspot in the field of soil ecology, such problems as the nondestructive techniques for collecting in-situ and the identification of allelochemicals from plant root exudates should be emphasized in the future.