Pulsed laser diode (LD) pumped passively Q-switched microchip laser is the most effective technique for miniatured large energy sub-nanosecond laser pulse generation. Here, the first pulse established time, time interval of the multi pulse, as well as the influence of the pump energy and pulse width on the output laser pulse number of the pulsed LD pumped passively Q-switched microchip laser are analyzed based on the rate equation theory. And YAG/Nd:YAG/Cr4+:YAG microchip laser pumped by a pulsed LD is established on this basis, in which the near-diffraction limit sub-nanosecond pulse laser at 1 064 nm is demonstrated with the single pulse energy of 1.2 mJ, the pulse width of 574 ps, the peak power of 2.1 MW, and the beam quality factor of M2=1.21.