This paper focuses on the modification and improvement of Ad hoc On-demand Multipath Distance Vector (AOMDV) in Mobile Ad hoc NETwork (MANET). An improved algorithm is proposed, aiming to maximize the benefit of multipath algorithm to achieve better performance metrics in both packet loss ratio and average delay. The main contributions of the paper are: (1) Factors that could cause packet loss and delay in MANET are analysed; (2) An improved algorithm: Ad hoc On-demand Multipath Distance Vector routing with Backup route Update (AOMDV-BU) is proposed, and the basic idea is demonstrated; (3) Several groups of NS-2 simulations with 40 nodes are conducted to validate the improvement of the proposed algorithm against the original AOMDV; (4) Performance comparison between AOMDV and AOMDV-BU are analysed in detail, showing that AOMDV-BU has achieved at most 20\% improvement in average end-to-end delay as well as at most 40\% improvement in packet loss ratio in certain scenarios.