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The spectral broadening of motional Stark effect (MSE) is simulated in the HL-2A tokamak,which is in good agreement with the experimental observation.The divergence angle of the neutral beam,finite lens size and beam energy dissociation are the main broadening factors,while the lens size is the most important one.The experimental spectra fitted by multiple Gaussian distributions reveal that the Stark splitting and its full width at half maximum (FWHM) are ～0.07nm and ～0.19nm,respectively.Therefore,the π and σ components of the MSE will overlap since the Stark splitting is much smaller than its spectral broadening.A narrowband filter with the FWHM of about 0.16nm is good enough to isolate the two components for providing the polarization fraction over 50％,which guarantees reasonable measurement precision for magnetic field pitch angle.