The high-power femtosecond laser has now become an excellent scientific tool for the study of not only relativistic laser–matter interactions but also scientific applications. The high-power femtosecond laser depends on the Kerr-lens modelocking (KLM) and chirped-pulse amplification (CPA) technique. An all-Ti:sapphire-based 30-fs PW CPA laser, which is called the PULSER (Petawatt Ultrashort Laser System for Extreme Science Research) has been recently constructed and is being used for accelerating the charged particles (electrons and protons) and generating ultrashort high-energy photon (X-ray and γ-ray) sources. In this review, the world-wide PW-level femtosecond laser systems are first summarized, the output performances of the PULSER-I & II are described, and the future upgrade plan of the PULSER to the multi-PW level is also discussed. Then, several experimental results on particle (electron and proton) acceleration and X-ray generation in the intensity range of mid-1018 W/cm2 to mid-1020 W/cm2 are described. Experimental demonstrations for the newly proposed phenomena and the understanding of physical mechanisms in relativistic and ultrarelativistic regimes are highly expected as increasing the laser peak intensity up to over 1022 W/cm2 ∼1023 W/cm2.