“Electron trapping” is the main acceleration mechanism in the laser wakefield acceleration, and the theoretical analysis indicates that more electrons are trapped when the initial velocity of the electrons are increased. A new wakefield acceleration scheme named “triangle-shaped laser pulse wakefield acceleration” is proposed and tested by 2D3V PIC simulation. The slow-rising part of the triangle-shape will excites stimulated Raman scattering(SRS) which preheats electrons while the sharp-descending part of the pulse drives the wakefield to trap more electrons. Our simulation also indicates that the side stimulated Raman scattering (SSRS) is the main instability when the length of the incident laser pulse is greater than the plasma wavelength. Furthermore, the electrostatic wave mode with the greatest growth rate in SSRS is always the one with the largest wave vector kp=(2ωp/ω0)1/2, and the scattering angle satisfies θ=cos-1(2ωp/ω0).