The magnetic anomaly associated with the premartensitic transtormation (PT) in Heusler alloy Ni2MnGa is investigated by using the Greens function technique based on the magnon-TA phonon interaction.The results of the numerical calculations suggest that the magnetic anomaly during the PT may originate from the strong magnon-TA phonon interaction,because this kind of coupling effect will destroy the magnetic ordering in some local zones and reduce the magnetization of the system.Magnon-magnon interaction plays a positive role in enhancing the magnetization and tends to reduce the PT temperature while this kind of interaction has little effect on the characteristic TA phonon-wavevector corresponding to the anomaly of spontaneous magnetization at the PT temperature.