A novel all-fiber ultra-long ring cavity erbium-doped fiber laser is reported. The laser consists of only single mode fiber (SMF) components except the nonlinear polarization rotation (NPR) parts which are utilized to achieve self-starting passively mode-locking. The total length of the ring cavity amounts to 4.046 km, which is achieved by incorporating a 4 km SMF. In experiment, the laser generates stable mode-locked pulses with a 50.92 kHz fundamental repetition rate. The maximum average power of output pulses is 2.73 mW, which corresponds to a single pulse energy of 53.61 nJ.