Secret keys play a critical role in secure data transmission, and an efficient secret key generation scheme will enhance the secure transmission rate. However, the secret key is a scarce resource for classical encryption schemes like the Rivest-Shamir-Adleman (RSA) cryptosystem and its counterpart in quantum applications, and we should extract it from all possible random sources to maximize the key rate. In this paper, we investigate the achievable rate of the secret key generation (SKG)-channel-model based on an actual measured multi-mode fiber (MMF) channel, where there is an additional public discussion channel between the legitimate parties, which the adversary can perfectly overhear. In particular, we measure the fiber transmission matrix by digital holography. Based on the real measurements, we show that for 55-mode MMF, we can achieve a positive secret key rate at 51.3 bit/channel use with mode-dependent loss (MDL) at 1 dB and transmit power at 20 dBm. This demonstrates the feasibility of SKG in MMF.