中文摘要：

Blazar 3C 345的红移以及从射电到X射线波段的能谱等方面与伽玛射线源3C279很相似，但是EGRET却没有探测到来自3C345的辐射．从光变幅度（8GHz、22GHz、37GHz和B波段），光学最小光变时标和不同波段间的时间延迟3个方面，比较3C345与伽玛射线源3C279和3C273的异同，寻找3C345没有伽玛射线的可能原因．分析结果显示；光变幅度，在射电波段，3C345与3C279的更相似，在光学波段，3C279、3C345和3C273依次递减并观测到了天量级的光变；还发现3C345的时延要比3C279长很多，而与3C273相近．基于3C345与3C273的其他观测特征的相似性，如都观测到了大兰包，红外光度相当，那么3C345的伽玛辐射能谱可能与3C273相似，伽玛光度也相当．简单计算表明，若3C273处于3C345的红移处时，即使处在爆发态EGRET也探测不到3C273，这可能正是EGRET从未探测到3C345的原因．

英文摘要：

The blazar 3C 345 is pretty similar to the gamma-ray blazar 3C 279 in its redshift and the spectrum from radio to X-ray band. However, the EGRET had not detected its radiation. The causes of the 3C345 without gamma-ray are studied and analyzed by comparing the differences and similarities among variability amplitude, the optical shortest variability timescale and the time lag among variations in optical B band and radio band （8 GHz, 22 GHz and 37 GHz）. In variation amplitude aspect, the researching results indicate that 3C 345 is similar to 3C 279 in radio band and in optical variability amplitude, 3C 279 is the largest one, 3C345 is much less than 3C 279 and the 3C273 is the least one. All of the three objects have been detected variability on the order of days in optical band. Besides, the time lag of the 3C 345 is longer than that of 3C 279, while approximately as long as that of 3C 273. It seems that the gamma-ray spectrum of the 3C 345 was probably similar to 3C 273 and their gamma-ray luminosity was almost equal. According to the other similarities, such as both having been detected the big blue bump and the similar infrared luminosity in 3C 345 and 3C 273, we roughly amounting indicate that 3C 273 would not be detected even at the state of burst if 3C273 moved to the place of 3C345,which might be the reason why 3C 345 has not been detected by EGRET ever since.