The present invention relates to a system for pretreating a test sample in order to analyze a test sample containing suspended solids in an on-line analyzer can, and more particularly to an ultra filtration system for an on-line analyzer for filtering suspended solids in order to stably quantitatively analyze a test sample containing a large amount of suspended solids using a module-type hollow fiber membrane filter intake system in an online analyzer. The ultra filtration system according to the present invention may be useful to obtain a reliable analysis result of a test sample using a filtered water in the field of environments, foods, chemistry, microorganisms, etc. by continuously separating solids from a liquid test sample containing a large amount of the solids and optimizing the test sample, in order to analyze the test sample in an on-line analyzer, and also to sense and swiftly respond to the real-time change in process and to minimize unnecessary manpower wastes, for example, extending a life span of an on-line analyzer, and maintaining, examining and correcting the analyzer, etc.