A study of the carrier transport performance of GaAs/AlGaAs very long wavelength quantum well infrared photodetectors (peak wavelength: 15 μm; period: >40) has been carried out based on quantum wave transport theory. It was shown that the thermoexcitation effect dominates in the devices. By the current continuity and self-consistent calculation based on Schrdinger equation and Poisson equation, we found that the main feature resulting from the model is the redistribution of the carriers and the electric field along the whole structure being made uneven to maintain current conservation. The high-field region extends over a few barriers near the emitter contact, which takes up a considerable part of the applied voltage. The conventional flat band model assumes that the applied voltage drops linearly across the structure, leading to the numerical value of dark current to be far from the experiments, especially at small bias. The numerical result of self-consistent calculation well explains the measurement data.