In this paper, the polarization absorption and output characteristics and the multi-wavelength laser operation characteristics of 10 at.% Er∶YAP crystal are studied. The polarization absorption spectra of Er∶YAP crystal is measured for the first time, the result indicated that the maximum absorption coefficients of crystal to linearly polarized light parallel to axis a and axis c in the absorption peak corresponding to the 4I11/2 energy level are 4.606 3 cm-1 (at 975.2 nm) and 2.936 6 cm-1 (at 967.6 nm), respectively. Therefore, the absorption efficiency of Er∶YAP crystal to pump light can be improved by choosing linearly polarized pump light with properly wavelength, which is conducive to improving the laser performance. In the xenon lamp pumped laser expeiments, four spectral lines of 2 710, 2 728, 2 795 and 2 918 nm are realized laser output under the free operation condition, the polarization and threshold characteristics of each spectral line are studied respectively. The laser output of single wavelength of 2 918 nm, four spectral lines of 2 710, 2 821, 2 837, 2 862 nm and single wavelength of 2 710 nm are achieved by adding JGS quartz plate, mica plate and K9 lens selective absorber respectively in the resonator cavity. The laser spectra are measured under the conditions of free operation and different selective absorbers, As well by contrasting with the transmittance spectra of different selective absorbers and the previously reported fluorescence spectra, it is proved that the oscillation is starting spectrum lines in Er∶YAP crystal could be selected by adjusting the resonator cavity. The experimental results show that the polarization direction of 2 918 nm spectral line is parallel to axis c and the polarization direction of 2728 nm spectral line sometimes parallel to the axis c and sometimes parallel to the axis a, besides, the polarization direction of all other spectral lines like 2 710, 2 795, 2 821, 2 837 and 2 862 nm are parallel to the axis c. The four laser spectral lines of 2 710, 2 728, 2 750 and 2 795 nm are obtained under LD end-pumping condition, in which the spectral line of 2 750 nm is realized laser output in Er∶YAP crystal for the first time. All these four spectral lines are linearly polarized light with polarization direction parallel to the c axis of the crystal. In addition, the absorption spectrum of Er∶YAP crystal is measured in 8 K low temperature, the spectrum was peak fitted by Gauss function, according to the fitting results the stark level of the upper and lower level are identified. And combining the laser spectra and fluorescence spectra, the possible laser spectral line transitions are also recognized. In this paper, the research of the Er∶YAP crystal polarization characteristics and multi-wavelength laser operating characteristics have instructive significance for the realization of Er∶YAP crystal Q-switching technique and the selection of electro-optic Q-switching crystal.