The application of garnet-type solid electrolytes in all-solid-state Li metal battery is seriously impeded by the lithium dendrite and resultant short-circuit. To explore its growth mechanism, in this work, a special asymmetric cell configuration is designed with a porous Li_6.55La_2.95Ca_0.05Zr_1.5Ta_0.5O₁₂ (LLCZTO) electrolyte between two mutually perpendicular Li metal strips. Besides the generally perceived lithium dendrite growth through grain boundaries, it is witnessed for the first time that lithium dendrite could also penetrate and break the LLCZTO grains, i.e., intragranular growth. Furthermore, uniform distributed lithium spherical particles, instead of lithium dendrite, are found on the surface of LLCZTO with Au-coating, demonstrating the beneficial effect of Au layer on restraining lithium dendrite growth. These findings shed new light on the growth mechanism and inhibition measures of lithium dendrite and would be helpful for the future research of all-solid-state Li metal battery with garnet-type solid electrolytes.