Polyethyleneglycol (PEG) is nontoxic, nonimmunogenic, hydrophilic, chargeless and nonbiodegradable poliymer. Its usage as a part of therapeutics protein drugs is common in medicine practice. It is known that covalent attachment of PEG conduces to prolong blood circulation half-lives, improves drug solubility and stability and reduces immunogenicity. It allows optimizing pharmacodynamic and pharmacokinetic drug properties. The goal of structure optimization of therapeutic proteins conjugates with PEG is to reduce loss of biological activity. It can be reached through controlled site- specific pegylation. We introduce two-step modification of proteins with branched polyethylenglycols via click-chemistry, synthesis of branched PEG azide reagent on the base of tris(hydroxymethyl)aminomethane with three linear PEG polymers. At first, we introduce alkyne groups with NHS-ester of alkyne acid in BSA protein. Then, branched PEG azide reagent reacts with alkyne function via CuAAC. Purification of the conjugates was done via gel-chromotography. Number of modifications was calculated from MALDI mass-spectra.