The invention discloses a reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay kit and a detection method for grass carp reoviruses (GCRV), and belongs to the technical field of the detection of fish viruses. The RT-LAMP assay kit for the grass carp reoviruses comprises the following ingredients: 1*ThermoPol Reaction Buffer, Bacillus stearothermophilus deoxyribonucleic acid (BstDNA) polymerase, deoxyribonucleotide triphosphate (dNTPs), AMV reverse transcriptase, outer primers (F3 and B3), inner primers (FIP and BIP), Betaine, MgCl2, dithiothreitol (DTT) and 1,000*SYBR Green I. The assay kit has the characteristics of simpleness, convenience, quickness and high specificity and sensitivity; GCRV-104 in a sample can be accurately detected within 2 hours only by using a water bath kettle or metal bath; and the assay kit can detect grass carp tissues infected by the GCRV-104 and cells (such as cytokine-induced killer (CIK) cells) infected by the GCRV-104, and is applicable to the on-site rapid detection of the GCRV-104.