The present disclosure provides a method for preparing an interlayer insulating layer and a method for manufacturing a thin film transistor, and a thin film transistor, belongs to the field of display technology, and can solve the problem of poor resistance to breakdown of the interlayer insulating layer in the related art. The method for preparing an interlayer insulating layer includes the following steps: forming a silicon oxide layer with a first reaction gas and forming a silicon nitride layer with a second reaction gas such that hydrogen content in the silicon nitride layer is less than or equal to hydrogen content in the silicon oxide layer.