Grain length is a directly factor affecting rice yield, and it also impacts on rice quality. In order to exploit and isolate more genes controlling grain length, and applies these genes into breeding practice for increasing rice yield and improving rice quality. In this study, an F2 isolated population was constructed derived from a cross between a dwarf wild rice mutant with short-grain and a cultivar variety KJ01 with long-grain. And then, a series of experiments were conducted, including of genetic analysis for grain length of the F2 population, and QTL mapping, and major QTLs analysis by using IciMapping V3.0 software. The results are as follows: The grain length was a quantitative trait controlled by multiple genes; By using 132 pairs of polymorphic molecular markers evenly distributed on 12 chromosomes of rice, QTL linkage analysis was performed on 543 F2 individuals, and a linkage map of 1 713.94 cM controlling rice grain length was constructed; 24 QTLs were detected by linkage analysis, only 3 QTLs showed additive genetic effects, and the others showed negative genetic effects; Three major QTLs were identified and located in interval markers of PSM379~RID24455, RID24455~RM15689, and RM571~RM16238 on chromosome 3, which explained phenotypic variation for 54.85%, 31.02%, and 7.62, respectively. Among them, no cloned gene controlling grain length has been reported between PSM379~RID24455, which was verified as a novel major QTL. Our results would provide a theoretical basis for cloning the novel grain length gene and molecular breeding of grain shape in rice.