The pyrolytic product vapor of Helianthus annuus stems was analyzed by Pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS) using the internal standard (ISTD) method with different pyrolysis temperatures and times. 1,3,5-tri-tert-butylbenzene (TTBB) was found to be the best ISTD chemical in this study. Scanning electron microscopy (SEM) revealed that, for the solid-state product, the pores and mesh structure gradually increased along with the pyrolysis temperatures and time. Sintering and porous destruction were observed at a lower pyrolysis temperature (600 °C) with longer time (0.5 min). The pyrolysis vapors contained small gas molecules such as CO2 as well as complex organic compounds, mainly alcohols, esters, acids, aldehydes, ketones, aromatic compounds, etc. In these products, aldehyde, ketone, and aromatic compounds were the main biochemicals; the appropriate pyrolysis temperature to produce aldehydes and ketones was 700 °C, and 600 °C was suitable for aromatic compounds. The regularity of the distribution of products and pyrolytic conditions was explored through eight representative compounds. The relationship between the product contents and pyrolysis conditions were complex for Helianthus annuus stems, but partial least squares discriminant analysis (PLS-DA) methods were a powerful tool for screening biochemicals whose absolute contents were sensitive to the pyrolysis conditions.