The invention provides a preparation method of cyano-alkyl and polyether co-modified polysiloxane. The method includes the following steps that cyano-alkyl silane is hydrolyzed, cyano-alkyl silane hydrolyzate is obtained, an alkali catalyst is added into the cyano-alkyl silane hydrolyzate, a polyether silicon oil and cyclosiloxane under protection of inert gas, and reaction is performed at 80-130 DEG C; the cyano-alkyl and polyether co-modified polysiloxane is prepared; the content of cyano groups in the cyano-alkyl and polyether co-modified polysiloxane is 0.05-5 mmol/g, the molecular weight of the polyether silicon oil is 500-100,000, the number of EOs in the polyether silicon oil is 3-100, and the number of POs in the polyether silicon oil is 0-40. The cyano-alkyl and polyether co-modified polysiloxane prepared through the method can serve as auxiliaries for producing high-performance carbon fibers. Particularly, the cyano-alkyl and polyether co-modified polysiloxane can serve as silicon oil agents for producing carbon fibers and can effectively moisten the surfaces of the fibers, reduce friction between the fibers and friction between the fibers and a guide roller, reduce surface defects and electrostatic accumulation caused by friction, and effectively prevent hot-melting adhesion of precursor fibers at the pre-oxidization stage and at the low-temperature carbonization stage.