Abstract: To address the challenges of multi-timescale fluctuations in renewable energy output and carbon emissions in integrated energy systems, this paper proposes a low-carbon economic planning method for an electricity-hydrogen-heat integrated energy system. First, considering the cross-day continuous variation characteristics of energy in power-to-hydrogen long-duration energy storage, an operation model that accounts for energy linkage between typical days is developed. This model, combined with battery storage, resolves the multi-timescale power and energy balance problem in the context of renewable energy generation. Second, a negative emission technology, namely biomass energy with carbon capture and storage (BECCS), is introduced, which participates in both electricity and carbon markets. A planning model for the integrated energy system is then constructed, incorporating the negative emission technology and its market participation. Simulation results demonstrate that, compared with conventional models, the proposed approach can reduce the total system cost by 19.6% and cut carbon emissions by 85.41%, effectively achieving a balance between economy and low-carbon performance.