Spike density and processing quality are important traits during the evolution of wheat, which is controlled by multiple gene loci. The associated gene loci have been heavily studied with slow progress. A common wheat mutant with extremely compact spikes and good processing quality was isolated. The gene (Qc1) responsible for the mutant phenotype was mapped and cloned, and the cellular mechanism for the mutant phenotype was investigated. Qc1 originated from a point mutation that interferes with the miR172-directed cleavage of the Q gene, leading to its overexpression. Qc1 reduces the longitudinal cell size of rachises, resulting in an increased spike density. Qc1 increases the number of vascular bundles, which suggests a higher efficiency in the transportation of assimilates in the spikes of the mutant than in the WT. This could account for the improved processing quality. The effects of Qc1 on spike density and wheat processing quality were confirmed by the identification of nine common wheat mutants having four different Qc alleles. These results deepen our understanding of the key role of Q gene, one of the most important domestication gene for wheat, and provide new insights for the potential application of Qc allele in wheat breeding.