Large core diameter crystal waveguides can absorb higher power pump light and achieve higher output power. At the same time, the peak power density in the core layer is relatively low during mode-locking operation, and the accumulation of nonlinear effects is reduced. Based on this, a passively mode-locked picosecond laser based on Yb∶YAG large core diameter crystal square waveguide is constructed. In the experiment, firstly, the high mirror is used instead of the semiconductor saturable absorption mirror (SESAM), and the position and angle of the crystal waveguide are adjusted to match the pump light with the waveguide core layer under higher pump power. Then, the angle of the spherical mirror is carefully adjusted to couple the signal light into the waveguide core layer to minimize the loss in the cavity. The designed laser adopts a folded cavity structure, and achieves an average power of 10.2 W, pulse width of 65 ps, repetition rate of 30.15 MHz, and single pulse energy of 0.34 μJ without dispersion compensation device.