Nowadays, the integration of blockchain and MANET improves both MANET security and blockchain scalability. Hashgraph can better support the MANET-based blockchain by its inherent features, i.e., gossip about gossip, Byzantine fault tolerance, etc. In order to understand the limitations and performance of hashgraph in MANETs, this work investigate the theoretical model with two metrics. The metrics include the degree of node isolation and witness confirmation delay. By using the differential equations and markov chain, this work quantitatively analyze the general hashgraph with an optimal contact probability and the improved hashgraph resisting eclipse attacks, as well as identifying the expressions of these performance metrics. Extensive experiments support the analysis and reveal the impact of moving nodes and Byzantine nodes on the hashgraph performance in MANETs. To the best of our knowledge, our work is the first to apply DAG (Directed Acyclic Graph) blockchain for the MANETs from a methodological perspective.