“Yu (于)” and “Yu (於)” both exist in The History of the Three Kingdoms, while the frequency of usages are different, with same and different usages. This paper takes The History of the Three Kingdoms During Shaoxi Period of Song Dynasty (The Twenty-Four Histories in Collection of Various Editions) as the research material, and investigates the usages of the words “Yu (于)” and “Yu (於)” in it, also referring to the other three editions of The History of the Three Kingdoms and checking each other to clarify the words and usages. In The History of the Three Kingdoms, “Yu (於)” appears 4544 times, “Yu (于)” appears 861 times, and the usage ratio is 5.28:1. “Yu (於)” is more widely and frequently used than “Yu (于)”. These two words are used most abundantly as prepositions. The prepositions “Yu (于)” and “Yu (於)” can both guide the complements of time and place, as well as elicit action objects and express passiveness. Additionally, in the three pairs of common phrases, the usages and meanings of the corresponding words are same. The difference is that the preposition “Yu (於)” has more plentiful usages and fixed usages of “Yushi (於是)” and “Zaiyu (在於)”, moreover, “Yu (於)” is often used after war verbs, and “Yu (于)” is rarely used. Furthermore, “Yu (於)” can be used as an interjection, and “Yu (于)” in proper nouns cannot be written as “Yu (於)”. It can be seen that, different from the pre-Qin period, the grammatical function of “Yu (於)” in The History of the Three Kingdoms is much stronger than that of “Yu (于)”, which reflects the trend that the weakening of the usages of “Yu (于)” in the Wei, Jin, Southern and Northern Dynasties, while the function of “Yu (於)” is gradually becoming stronger. Besides, there are many usages preserved through “Yu (於)” from ancient Chinese to modern Chinese.