An ultrahigh performance liquid chromatography coupled with quadrupole－orbitrap mass spectrometry（UHPLC－Quadrupole－Orbitrap MS） was developed for the determination of cyromazine and its metabolite melamine in pig urine samples．The pig urine sample（2.0 g） was ultrasonically extracted with 1% trichloroacetic acid under the pH value of the extraction mixture lower than 3，followed by purification with a mixed cation solid phase extraction column（MCX），then the eluent from the MCX column was blow dried under nitrogen flow，and finally reconstituted with 95% acetonitrile aqueous solution．The reconstituted solution 10 times diluted was injected into the UHPLC－Quadrupole－Orbitrap system using a Syncronis HILIC column（100 mm × 2.1 mm，1.7 μm） at 35 ℃，with 10 mmol/L ammonium acetate and acetonitrile as the mobile phases．The mass spectrometer was operated in positive electrospray（ESI + ） and parallel reaction monitoring（PRM） mode．Qualitative analysis was carried out according to the accurate masses of each target compound，its fragment ions，and the abundance ratios of the fragment ions．The external standard method and internal standard method were used for the quantification of cyromazine and melamine，respectively．Method validation showed that there were good linear relationships for the two targets in the range of 0.2－50.0 ng/mL，with their correlation coefficients（,r,2,） larger than 0.99．At three spiked levels of 1.0，2.0 and 150.0 μg/kg，the recoveries for cyromazine and melamine in pig urine sample ranged from 78.0% to 105%，with relative standard deviations（RSD） no more than 11%．The same spiked samples were used for comparison between PRM of orbitrap and multiple reaction monitoring（MRM） mode of triple quadrupole．The results obtained exhibited no significant difference，indicating that both methods have good quantitative ability for determination of cyromazine and melamine in pig urine sample．With good recovery，dynamic range and quantification accuracy，the developed method could be applied to the simultaneous qualitative and quantitative analyses of cyromazine and melamine residues in pig urine samples．