The invention relates to a reagent kit and application thereof, particularly relates to an isothermal amplification reagent kit for detecting fever and thrombocytopenia syndrome viruses and application of the isothermal amplification reagent kit, and belongs to the field of biotechnologies. The isothermal amplification reagent kit for detecting the fever and thrombocytopenia syndrome viruses contains oligonucleotide and comprises RNA (ribonucleic acid) extraction reagents (1), isothermal amplification reaction liquid (2), positive control templates (3) and negative control components (4). The isothermal amplification reaction liquid (1) comprises 1*Bst Buffer, betaine, MgSO<4>, dNTPs, RNase inhibitors, Bst DNA (deoxyribonucleic acid) polymerase, AMV reverse transcriptase, sterile deionized water, upstream and downstream strand displacement primers, upstream and downstream probes and a cross amplification primer; the positive control templates (3) are in-vitro transcription products of M gene segments of the fever and thrombocytopenia syndrome viruses; the negative control components (4) are sterile deionized water. The isothermal amplification reagent kit and the application have the advantages that the isothermal amplification reagent kit is high in detection sensitivity, specificity and repeatability, simple in operation step and short in reaction time, and has a broad application prospect, integral reaction procedures can be carried out at the same temperatures, the fever and thrombocytopenia syndrome viruses can be detected only by the aid of a single isothermal instrument, and the like.