A heatwave (HW) is a spatiotemporally contiguous event that is spatially widespread and lasts many days. HWs impose severe impacts on many aspects of society and terrestrial ecosystems. Here, we systematically investigate the influence of the selected threshold method (the absolute threshold method (ABS), quantile-based method (QTL), and moving quantile-based method (QTLmov)) and selected variables (heat index (HI), air temperature (Tair)) on the change patterns of spatiotemporally contiguous heatwave (STHW) characteristics over China from 1961–2017. Moreover, we discuss the different STHW change patterns among different HW severities (mild, moderate, and severe) and types (daytime and nighttime). The results show that (1) all threshold methods show a consistent phenomenon in most regions of China: STHWs have become longer-lasting (6.42%, 66.25%, and 148.58% HW days (HWD) increases were found from 1991–2017 compared to 1961–1990 corresponding to ABS, QTL, and QTLmov, respectively, as below), more severe (14.83%, 89.17%, and 158.92% increases in HW severity (HWS) increases), and more spatially widespread (14.92%, 134%, and 245.83% increases in the summed HW area (HWAsum)). However, the HW frequency (HWF) of moderate STHWs in some regions decreased as mild and moderate STHWs became extreme; (2) for threshold methods that do not consider seasonal variations (i.e., ABS and QTL), the spatial HI exceedance continuity was relatively weak, thus resulting in underestimated STHW characteristics increase rates; (3) for different variables defining STHWs, the relative changing ratio of the HI-based STHW was approximately 20% higher than that of the Tair-based STHW for all STHW characteristics, under the QTLmov threshold; (4) for different STHW types, the nighttime STHW was approximately 60% faster than the daytime STHW increase considering the QTL threshold and approximately 120% faster for the QTLmov method. This study provides a systematic investigation of different STHW definition methods and will benefit future STHW research.