There are many valuable rare and unusual objects in spectra dataset of Sloan Digital Sky Survey (SDSS)Data Release eight (DR8),such as special white dwarfs (DZ,DQ,DC),carbon stars,white dwarf main-sequence binaries (WDMS),cata-clysmic variable (CV)stars and so on,so it is extremely significant to search for rare and unusual celestial objects from massive spectra dataset.A novel algorithm based on Kernel dense estimation and K-nearest neighborhoods (KNN)has been presented, and applied to search for rare and unusual celestial objects from 546 383 stellar spectra of SDSS DR8.Their densities are esti-mated using Gaussian kernel density estimation,the top 5 000 spectra in descend order by their densities are selected as rare ob-jects,and the top 300 000 spectra in ascend order by their densities are selected as normal objects.Then,KNN were used to classify the rest objects,and simultaneously K nearest neighbors of the 5 000 rare spectra are also selected as rare objects.As a result,there are totally 21 193 spectra selected as initial rare spectra,which include error spectra caused by deletion,redden, bad calibration,spectra consisting of different physically irrelevant components,planetary nebulas,QSOs,special white dwarfs (DZ,DQ,DC),carbon stars,white dwarf main-sequence binaries (WDMS),cataclysmic variable (CV)stars and so on.By cross identification with SIMBAD,NED,ADS and major literature,it is found that three DZ white dwarfs,one WDMS,two CVs with company of G-type star,three CVs candidates,six DC white dwarfs,one DC white dwarf candidate and one BL Lacer-tae (BL lac)candidate are our new findings.We also have found one special DA white dwarf with emission lines of CaⅡ triple and MgⅠ,and one unknown object whose spectrum looks like a late M star with emission lines and its image looks like a galaxy or nebula.%SDSS DR8海量光谱中包含许多有研究价值的稀有天体，如特殊白矮星（DZ，DQ，DC）、碳星、白矮主序双星、激变变星等，如何在海量光谱中自动搜寻稀有天体有着极其重要的意义。提出一种基于核密度估计和 K-近邻（K-nearest neighbor，KNN）相结合的方法在 SDSS DR8信噪比大于5的546383个恒星光谱中搜寻稀有天体。首先对光谱进行高斯核密度估计，选取概率最小的5000个光谱作为稀有类，概率最大的300000个光谱作为普通类，然后进行 KNN 分类，同时也将5000个稀有光谱的 K 个最近邻也作为稀有的天体，结果共有21193条光谱。为了方便分析，对这些光谱聚类后进行人工检查。这些光谱主要包括由于数据缺失、红化、流量定标不准引起的问题光谱、行星状星云、没有物理联系的光谱双星、类星体、特殊白矮星（DZ，DQ，DC）、碳星、白矮主序双星、激变变星等。通过和 SIMBAD，NED，ADS 及一些主要的文献交叉验证，我们新发现了 3个 DZ 白矮星、1个白矮主序双星、2个伴星为 G 型星的激变变星，3个激变变星的候选体、6个 DC 白矮星，1个 DC 白矮星候选体和 1个 BL Lacertae（BL lac）候选体。还发现了 1个有 CaⅡ三重发射线和 MgⅠ发射线的 DA 白矮星和 1个光谱上表现出发射线的晚 M 恒星但测光图上像是一个星云或星系。
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