We report a passively Q-switched all-fiber Yb-doped laser using the structure of step-index multimode fiber and graded-index fiber (SIMF-GIMF) as a new saturable absorber (SA) based on the nonlinear multimode interference (NL-MMI). Through bending the SA to a certain state, we obtain a stable Q-switched laser operation with the shortest pulse duration of 4.37 s and a corresponding repetition rate of 147.6 kHz with center at 1,039 nm. The maximum average output power and single pulse energy are recorded to be 2.409 mW and 16.3 nJ, respectively. To the best of our knowledge, this paper confirms experimentally for the first time Q-switched laser operation based on SIMF-GIMF SA in all-fiber lasers. In addition, we also observed unstable Q-switched mode-locking operation. All-fiber lasers based on SIMF-GIMF SA are attractive for practical applications and undergo no damage and performance degradation with time.