Chirped pulse spectral interferometer (CPSI) is an ultrafast single-shot diagnostic instrument, the large chirped pulses reference, pulse detection and accurate measurement of its spectral of electric field are critical factors to realize ultra-high time-resolution and large range of testing simultaneously. Based on the spectral interference between transform limit femtosecond pulse and chirped pulse, a linear optical measurement method for spectral phase measurement of large chirp pulse is proposed by theoretical analysis and numerical calculation. It is called asymmetric spectral interference method. Investigations show that when employing the asymmetric spectral interference method to measure the spectral phase of chirped pulses in CPSI, translation error of the time delays and relative smaller chirp of femtosecond pulse will produce a first order phase error. Consequently, the time-domain signal measured by CPSI will generate a time translation, but the relative time evolution of the measured signal is not affected.