Based on the urgent demand for multi-element coupled simulation and long-endurance capacity assessment of multi-capsule near-space airship， this paper describes six-degree-of-freedom position and attitude dynamics model， environmental thermodynamic model， inner-outer capsule thermodynamic model， and capsule helium loss model of multi-capsule near-space airships in detail. The dynamic and thermal coupling for the platform are realized through dynamic output information （e.g.， position， time， attitude and airspeed） and thermodynamic output information （e.g.， pressure forming volume and helium mass）， which can fully reflect the dynamic and thermal coupled rule of airships under atmospheric environment and the effect of operation， and reveals the quantitative long-endurance assessment ability for airships under the safety of overheat and overpressure. Through simulation， it is found that the maximum overheat of helium in the capsule reaches about 55 ℃ in the condition of floating with wind. To ensure the pressure safety of the internal and external capsule during the day， helium should be released actively， which leads to the flight altitude increase of approximately 100 m. At night， the pressure in the capsule will reach 0 Pa， and the buoyancy and the shape cannot be maintained， causing a rapid decrease of floating altitude，intensified attitude oscillation， and the floating duration of 26.5 h only. In addition， airships cannot achieve stable course flight with full power flight，and the airspeed inflow cooling cannot be maximized. A control system needs to be connected to achieve dynamic closed-loop route flight， so as to realize long-endurance flight， which has important engineering application value.