For the old product with the field using period, the single-stress & timing-conversion step-stress accelerated life test (SSALT) is carried out to derive the product's lifetime by using the field using data and accelerated life data. The product's lifetime is assumed to obeying to Weibull distribution with double parameters. Considering the stress difference between the field and laboratory, the lifetime distribution model based on correction factors is built. The SSALT data modeling analysis is carried out to build the cumulative distribution function under each stress. Data iteration algorithm is applied to obtain the estimates of unknown the model parameters. An example by Monte Carlo simulation test is analyzed to verify the accuracy and validity.