In order to improve the availability of the worst user location( WUL) algorithm, an im-proved algorithm is proposed.The shortcoming of low search efficiency of the worst user location al-gorithm is described, which severely restricts its application in integrity monitoring.To overcome the shortcoming of the traditional worst user location algorithm, based on the critical circle method, an improved critical cycle algorithm is proposed through revealing the relationship among nadir, orbit error vector and the worst user location and adding the geometric constraints of the worst user loca-tion search.Finally, GPS ( global positioning system) data is used as an example.Through numeri-cal example, it is shown that, when the tolerance is set to be 10 m, the search of the improved criti-cal circle method is performed 2 times.Compared with 28 times of the traditional critical circle meth-od, the proposed algorithm makes an improvement in search efficiency.%为了提高最差用户位置算法的可用性，提出了一种改进的临界圆算法。阐述了最差用户位置算法存在搜索效率低的缺点，严重制约了其在完好性监测中的应用。针对传统最差用户位置算法搜索效率低的缺点，在临界圆法的基础上，通过揭示了星下点、轨道误差向量与最差用户位置之间的关系，增加了最差用户位置搜索的几何约束，提出了一种改进的临界圆算法，提高了最差用户位置的搜索效率。最后以GPS为例，通过算例验证，在限差设为10 m的情况下，改进临界圆法对最差用户位置的搜索次数为2次，相较于传统临界圆法的28次，搜索效率上有了很大提高。
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