The invention provides a method for biosynthesis of a hydroxybutyric acid and hydroxycaprylic acid co-polymer from sludge. Aerobic domestication is performed on the activated sludge with sodium laurate used as a carbon source. Each domestication period is 12-36 h and adopts the running manner of water inlet, aerobic aeration, sludge discharging, sedimentation and water outlet. Preferably, the total time from stopping of aerobic aeration to re-aeration each time does not exceed 30 min. In each period, sampling is performed before sludge discharging, MLSS (mixed liquor suspended solids), the pH value and DO (dissolved oxygen) are tested, and after the values are stabilized, sludge domestication is completed. After sludge domestication is stabilized, the next step of a test for synthesis of the hydroxybutyric acid and hydroxycaprylic acid co-polymer from the sludge can be conducted. Production of a biochemical product with the higher added value from a cheap substrate, namely, the activated sludge, can be a new way for recycling of the sludge.