In this paper, we employ chaotic light as an entropy source, generated by semiconductor lasers with optical feedback induced by external cavity, to generate 500 Mbit/s random number sequence and then study specifically the influence of external cavity length on randomness. The results indicate that the generated random sequence has an weak periodicity caused by the external cavity feedback under a single-channel chaotic source, and can only pass two or three tests of the National Institure of Standards and Technology(NIST) when the external-cavity round trip delay time is an integral multiple of the sampling time. On the other hand, the weak periodicity can be eliminated by using two different chaotic sources when their external cavity lengths are unproportional to each other, and the obtained random sequence can pass all tests of the NIST after XOR processing.