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GRS 1915+105 is a galactic low-mass X-ray binary. Its energy spectra and black hole spin have been extensively studied. Since 2018 June, it has declined into a low-flux X-ray level, occasionally interrupted by multi-wavelength flares. Using the data of Insight-HXMT (Insight-Hard X-ray Modulation Telescope) satellite from August 30 to Oct 13, 2020, the energy spectra characteristics of GRS 1915+105 is investigated. The results show that the energy spectra can be well fitted with a Comptonized multi-temperature blackbody model. The evolution of the hardness-intensity diagram during the outburst remains in the soft state. Adopting new dynamical parameters of GRS 1915+105 (including the black hole mass M , the Sun mass M ⊙ , the inclination angle i , and the distance D): M = 12. 4 − 1.8 + 2.0 M ⊙ , i = 60 ∘ ± 5 ∘ , D = 8. 6 − 1.6 + 2.0 kpc, the group obtains a lower limit of the black hole spin, a * > 0.9990 , confirming that GRS 1915+105 is an extremely-spinning black hole. Considering the effects of a local absorber, the group adds the absorption component "tbpcf" into the model. Its equivalent hydrogen column density reaches 10 23 cm − 2 , which is consistent with the characteristics of "Compton-thick". [ABSTRACT FROM AUTHOR]