A method for relative quantitative analysis of a polymer by MALDI mass spectrometry, comprising the steps of: preparing a plurality of polymer specimens having a thickness variation of 30% or less by electrospray of a solution containing a polymer sample and a matrix through a mask for each concentration of the polymer sample; obtaining a MALDI mass spectrum by irradiating a laser to each of the plurality of polymer specimens; and preparing a quantitative calibration curve with a signal of a polymer sample from peak results of the MALDI mass spectrum, wherein the preparing the polymer specimen by electrospray through the mask comprises the steps of: (i) preparing an electrospray apparatus comprising a sample plate and an electrospray main nozzle and mounting the mask on the sample plate; and (ii) electrospraying the solution containing the polymer sample and the matrix onto the mask-mounted sample plate by the main nozzle, wherein the mask comprises a hole through which the polymer sample solution electrosprayed from the main nozzle passes onto the sample plate so as to obtain a polymer specimen having a small thickness variation on the sample plate, characterized in that the electrospray apparatus further comprises an auxiliary nozzle surrounding the main nozzle and coaxial with the main nozzle, and the electrospraying the polymer sample solution onto the sample plate by the main nozzle further comprises the step of spraying a solvent by the auxiliary nozzle to prevent the matrix from being deposited around the main nozzle. preparing a plurality of polymer specimens having a thickness variation of 30% or less by electrospray of a solution containing a polymer sample and a matrix through a mask for each concentration of the polymer sample; obtaining a MALDI mass spectrum by irradiating a laser to each of the plurality of polymer specimens; and preparing a quantitative calibration curve with a signal of a polymer sample from peak results of the MALDI mass spectrum, wherein the preparing the polymer specimen by electrospray through the mask comprises the steps of: (i) preparing an electrospray apparatus comprising a sample plate and an electrospray main nozzle and mounting the mask on the sample plate; and (ii) electrospraying the solution containing the polymer sample and the matrix onto the mask-mounted sample plate by the main nozzle, (i) preparing an electrospray apparatus comprising a sample plate and an electrospray main nozzle and mounting the mask on the sample plate; and (ii) electrospraying the solution containing the polymer sample and the matrix onto the mask-mounted sample plate by the main nozzle, wherein the mask comprises a hole through which the polymer sample solution electrosprayed from the main nozzle passes onto the sample plate so as to obtain a polymer specimen having a small thickness variation on the sample plate, characterized in that the electrospray apparatus further comprises an auxiliary nozzle surrounding the main nozzle and coaxial with the main nozzle, and the electrospraying the polymer sample solution onto the sample plate by the main nozzle further comprises the step of spraying a solvent by the auxiliary nozzle to prevent the matrix from being deposited around the main nozzle.