Pichia Pastoris is widely used in fields such as pharmaceutical proteins, biotechnology, and industrial enzyme preparations, and has high research value. In order to improve the yield and quality of exogenous protein expression in Pichia Pastoris, it is necessary to monitor the bacterial concentration. However, currently there is a lack of economically effective sensors for real-time monitoring of bacterial concentration values, and the industrial production process of Pichia Pastoris is prone to model failure due to changes in multiple operating conditions. In response to the above issues, this book proposes a multi-condition soft sensor modeling method to predict the bacterial concentration during Pichia Pastoris fermentation online. This book is not only a monograph of scientific research achievements, but also a new effective detection and control method for biological fermentation. Furthermore, it is hoped that it can enlighten readers in theoretical research and engineering application.