The invention discloses a middle separation wall top original lining cavity backfilling construction method when a single-hole tunnel is changed into multiple arches. The method sequentially comprises the following steps that a, a middle separation wall is cast; b, an anterior hole is constructed; c, hole slag dug from the anterior hole is used for filling back a cavity arranged at one side of a posterior hole to the digging outline part of the posterior hole; d, a steel bar net is paved on the top of the backfilled hole slag, and then, concrete is sprayed on the surface of the steel bar net; e, an end template is arranged along a tunnel longitudinal frame, in addition, the end template, the surrounding rock and the original lining are fixedly built through lining by using anchor bolts; f, a side template is erected at the digging outline part of the anterior hole; g, concrete is filled and is compactly vibrated and smashed; h, the posterior hole is constructed; i, the original lining of the existing tunnel in the anterior hole and the posterior hole is dismounted. The construction method has the advantages that the original tunnel top cavity backfilling problem is effectively solved, so the pressure of the surrounding rock is uniformly transmitted onto a support protection structure, the stress uniformity and the structure completeness of the lining can be ensured, and the loading capability and the durability are improved.