Recently, the measurement scheme of quantum dot qubit decocoherence quantized by the longitudinal optical (LO) phonon spontaneous emission rate has attracted the attention and discussion of many researchers. However, it is not difficult to see that the above-mentioned measurement scheme still has some insufficient and imperfect aspects that are to be studied urgently. Considering from the physical mechanism, the essence of the above scheme is to quantify the decoherence time of qubit by using the excited state decay time or excited state lifetime of the polaron. However, so far, there is little research on how the ground state decay time and ground state lifetime of two-state polaron affect the coherence of qubit. There is no doubt that this is an equally important research topic. This is because, firstly, for the coherence of the quantum state of polaron, both the decay of the excited state and the decay of the ground state will destroy or attenuate the qubit coherence, secondly, the transition rate of the two-state polaron from the ground state to the excited state after absorbing an LO phonon is also a function quantifying the qubit decoherence time of two-state system of which the inverse is called the ground state decay time or the ground state lifetime. It may be called a measure of qubit decoherence time quantized by the ground state decay time or ground state lifetime of polaron. In this article, the ground-state and excited-state energy and wave function of the magnetopolaron in a donor-center quantum dot with asymmetric Gaussian potential are derived by Lee-Low-Pines transformation and Pekar-type variational methodd, and then the two-level structure for a qubit is constructed. The measure of qubit decoherence time of quantum dots quantified by ground state decay time of two-state polaron is established, which is compared with the well-known measure of qubit decoherence time of quantum dots quantified by polaron excited state decay time, and their physical mechanisms are revealed. By studying the influence of dielectric constant ratio, electro-phonons coupling constant, temperature and electromagnetic field on the ground state lifetime of magnetopolaron in the donor-center quantum dots with asymmetric Gaussian potential, the influences of material properties, temperature, electromagnetic field and other environmental factors on qubit decoherence of quantum dots are revealed, thereby revealing the mechanism of qubit decoherence caused by LO phonon effect.