diff --git a/src/chart/line/LineView.ts b/src/chart/line/LineView.ts
index 10b0b26bd03329942cb02fc6b5d6bdcde2c2db54..296e991c3b2054f2bedbefbdbe110a184777cd67 100644
--- a/src/chart/line/LineView.ts
+++ b/src/chart/line/LineView.ts
@@ -738,7 +738,7 @@ class LineView extends ChartView {
 
         if (polyline.style.lineWidth > 0 && seriesModel.get(['emphasis', 'lineStyle', 'width']) === 'bolder') {
             const emphasisLineStyle = polyline.getState('emphasis').style;
-            emphasisLineStyle.lineWidth = polyline.style.lineWidth + 1;
+            emphasisLineStyle.lineWidth = +polyline.style.lineWidth + 1;
         }
 
         // Needs seriesIndex for focus